These are some ideas I've collected

Vistaru, Lord of the Mountain, is attacking small towns surrounded by his kingdom in hopes of expanding his army before his assault on the capital.
Your party is sent to kill an evil Duke/King/Emperor etc.
Your party is pursued by a murder of deadly crows with needle sharp beaks.
Drugged and put on a boat in the middle of the ocean.
Trapped on an island with monsters and little food or water.
Soldiers in an army marching into unholy land.
Spies for an army.
Part of an exploration team.
Plain old surviving - Bandits, highwaymen, monsters, women, etc.
Portal from one plane has been opened and elemental monsters are invading a peaceful land.
Priest is kidnapped by opposing clerics.
Cult becomes angry with rival gang and attacks them.
A mysterious mound threatens a military base.
A rival country threatens war, peace must be obtained.
A Wizard hires you to retrieve magical artifacts from the ruins of an ancient city.
Locals hear chants in the nearby ruins.
Undead threaten cities across the nation.
A queer lightning storm wipes out an entire city - the king orders you to investigate.
An encrypted stone tablet is discovered during an archaeological expedition.
Several major politicians have been assassinated.
A large gang/terrorist organization wages war against a local city.
Barbaric humanoid ttribes threaten a supply post which boasts the majority of a country's natural resources.
The party comes across a gladiator stadium and is offered money in return for fighting.
Dragon blood is needed to cure a plague and your group is sent to collect some.
A powerful bard hires your party to retrieve a fabled magical instrument from a dead wizard's coffin.
A huge drought arises and you must travel to the Water Plane and battle elementals in order to direct a flow into the drying world.
Breaking into a dungeon/room/fortress etc. to rescue or capture a person or artifact.
The caracters must escort a dignitary.
A nation's enemies have a secret base that must be found.
The characters are imprisoned and must escape.
The ship the party is traveling on is hijacked.
Safari; characters must hunt down a creature.
Mysteriously, an entire city has lost its inhabitants overnight.
The characters must be counterspies and seek enemy agents.
The party competes in a sport/competition/game.
The characters must chase escaping bandits through the countryside/are chased by bandits through the countryside.
Discovering a portal to another plane leads to an exploration party.
A group of Doppelgangers have assassinated the town council and assumed their identities.
A medusa is constructing a garden of statues using townsfolk.
Statues in a large cemetery come to life at night.
A lich is raising a massive undead army.
A wizard has been kidnapped and his book of immortality/whatever is hidden in his tower, wanted by rival guilds.
A group of warriors has challenged the party to a match, to prove who is the strongest.
A thief has stolen one of the player's main pieces of equipment - spellbook, sword, divine focus, etc.
Demons are appearing all over the kindom. Why?
A giant believes that all humans exist for his dining pleasure.
A man with one eye is being hanged for "looking sinister."
A wizard has declared himself ruler of all he surveys - meaning most of everywhere.
The well has turned black.
Party members find a chained and punished god in an ancient, forgotten temple (Baseline for Andru Baleye story arc?)
Catacombs have opened beneath the city, guarded by ancient undead who let no one pass.
The sacred spear of Lesh has gone missing.
There is a black cloaked man in a tavern, though he is dead. Everybody says they saw him walk in just fine.
An ogre regards a female party member as "The perfect mate."
A wandering rogue believes that one of the party is his long lost brother.
An spirit begs for help from an evil party member.
A god's weapon has fallen to earth and now there is a race to obtain it.
Suddenly an entire country turns black.
A party member discovers a great sword made of red metal that has the properties of adamantine, mithril, silver and cold iron, but can be wielded in one hand as if it were a longsword. What is it? (the godly weapon, perhaps?)
A party member's armour comes to life and tries to leave.
A tribe of trolls is trying to make peace with a nearby village.
The party meets several groups of travelers headed in the same direction, claiming to be going to something called "The Games." (change name? The Great Hunt?)
An earthquake that leaves magical glowing fissures in the ground shakes directly beneath a kindom's capital.
A previously rithteous and good god suddenly calls genocide on all that are not his followers.
A paladin is after a party member because of a case of mistaken identity.
An area has an uprising of twisted, demon-like animals. What has happened?
Everybody who buys a charm from a certain vendor begins experiencing the effects of a curse that causes a physical aspect to change (odd skin colour, longer legs, hair changing colour or falling out, etc.) So far, the effects have not been particularly terrible, but how long will that keep up?
A beastial race (Rat/Lizarmen, etc.) are killing everything from paladins to assassins. Whose side are they on?
A shady character asks you to smuggle some barrels of moonshine into a city.
We need the Demon-slaying sword is we are to overcome the oncoming siege. Travel to that indifferent neighboring country and negotiate with their leader for a loan of the sword.
A member of a party of adventurers is on the run from the law but the Duke has asked to see the party. The adventurer is welcome but the guards do no know this and the character must be smuggled into the city.
Facing a pack of werewolves becomes a grueling all-nighter as they use hit-and-run tactics to lure characters away from the group so they may overpower them with their numbers. (Perhaps the party is trapped within a remote cabin or rundown fort or ruin) A group of guards/civilians are all the party have to aid them.
Book passage on a ghost ship; at night the crew turn into ghouls and kill each other - and anyone else unfortunate enough to be aboard - while below deck, it grows even worse.
A ring of regeneration is a great find - until a character realizes what is is turning them into... can the party do what it takes to remove the curse in time?
All who come in contact with the waters of a holy fountain rot and eventually become zombies.
A mysterious Goblin raids the town at night, striking and seemingly vanishing without a trace.
A druid is accused of killing other druids.
A hidden pathway in an abandoned house leads to a lair of a terrible Demon. The Demon keeps "slave girls" where a friend of the party is, but the "slave girls" are secretly Demons in disguise.
One of the party members is offered a slave girl as a reward for a job well done. Another character - e.g. a Paladin - finds this outrageous and demands he let her go.
The party and some others are raiding a city when they are captured and forced to entertain the king in the gladiator's arena.
A large tribe of Orcs are planning a raid on an Imperial city.
A Mage summons a demon but is unable to control it and is possessed.
The world is hit by an asteroid that mutates anyone that touches it.
A large egg of some sort is discovered and the Orcs are coming for it. Will the characters try to hatch it?
A thick plague-inducing fog is slowly covering the countryside.
A Paladin goes mad, turning evil while still thinking he is good.
The characters are shrunk by an evil wizard and have to find a way to reverse the spell.
An illusion of peace and tranquility is projected over a town. The characters must escape and discover what is really happening.
The God of Time punishes a town/city/village for not worshipping him, making everything happen out of sequence.
All the local wild life is killing people; are the Druids to blame?
Everyone is turned into monsters and the characters are run out of town.
The characters are framed for a genocide they didn't cause.
While crossing a sea, the boat is sunk by a monster of some variety. The charcters are saved by merfolk but trapped on a deserted island miles from the mainland.
A dragon captures the party and takes them to her mountain lair to feed to her wyrmlings.
The characters complete a job but their employer doesn't want to pay them.
Monsters are attacking a castle; the party must prevent the monsters from taking the castle.
A large rock worm is attacking a large city. One of the characters must kill the rock worm before the city goes under.
A time warp descends upon an isolated village, setting everyone and everything back a century into the past.
Several of the king's tax collectors have disappeared after travelling to a particularly dangerous area.
A good cleric tries to establish a temple to his god in a hostile city, hoping to graduall convert the populace.
Suspicion arises that the inhabitant of a town are gradually being replaced by imposters.
Animals within a particular area are becoming monstrous and mutated.
Ships are disappering within a five-mile radius of ocean.
A phantom river barge sails past a small town once per month, leaving malevolent, supernatural occurences in its wake.
A local ruler is suspected of making a pact with a powerful devil or demon.
The Assassin's Guild is planning to kill the mayor.
A witch's curse is causing a farmer's crops and livestock to die.
A mysterious curse is settling on town after town, rendering almost all forms of magic ineffective - e.g. ice and wind magic - while greatly empowering any who make use of Black or Demonic magic.
A band of clerics is robbing the graves at a large cemetery, creating an army of undead creatures.
A secret vigilante society is inflicting barbaric punishments on "evil-doers."
The daughter of a nobleman has run off with a seedy, disreputable commoner.
A small community of neutrally-aligned Orcs tries to convince inhabitants of a nearby town that they mean them no harm.
A formerly good cleric is suspected of secretly worshipping an evil god.
About to be hanged for his crimes, a powerful bandit leader is sprung from his prison by his cohorts.
Kobolds occupy an abandoned mine, using it as a base from which to launch raids on Human settlements.
A Sorceress has lost her unique familiar and all efforts so far to reveal the creature's whereabouts have been fruitless.
The party is sent to deliver the ransom for a woman, who is being held by a gang of brigands.
The king's ambassador has disappeared while en route to a hostile country and is suspected of being a traitor.
A megalomaniacal Duke has sent his minions in search of a weapon that would make him nearly invincible.
The king, a just, good ruler, is somehow being manipulated by his new adviser.
A series of murders are being committed with the MO of an executed killer.
Clerics are stymied as to how to remove a bizarre desecration that has fallen upon their temple.
Strange, glowing runes appear on the door of the town hall, forming a riddle.
A local farmer is suspected of breeding hell hounds, after several of the beasts were spotted killing the cattle of other farmers.
A Kraken is destroying any ships that try to enter or leave a city's port.
During a break from traveling in a big city, the party wakes up to find all of the city's inhabitants missing and the gate locked...
A growing maelstrom near a rocky coast is sucking ships down beneath the waves.
The party members are hired as marshals to escort a powerful Fallen Paladin back to the city, where he is to stand trial.
Magical droughts are descending upon an increasing number of communities.
The king sends the party to parlay with an adjacent kingdom contemplating war.
Villagers are fed up with a group of troublesome mercenaries.
Dryads fight to preserve their forest against an encroaching Human settlement.
Messengers must travel through a haunted swamp to deliver a timely warning to the Queen.
Members of a coven have infiltrated the city's populace and are working their black magic.
A magical fire in town resists all efforts to extinguish it.
An ancient artifact must be retrieved from the bottom of the sea.
A Druid is framing a werewolf by committing crimes while in the form of a wolf.
Local bandits are far too well informed about trade caravans. A trade post keeper is suspected of being an informant.
A small town is being raided every few weeks by an unusually well organized group of Goblins and Hobgoblins. It turns out that a group of Goblin Shaman ordered these raids in order to draw out the best soldiers and mercenaries in the region before unleashing a massive army of Goblins, Hobgoblins, Bugbears, Ogres and Trolls.
A merchant comes to the characters to ask them to retrieve a chest that was stolen by Furbolgs. When the group finds the Furbolgs, they are told the merchant had been hiring mercenaries to hunt and kill them so he could trade their pelts.
A cursed orb serves to reduce intelligent lifeforms into feral states and enraging any animals that come upon it.
A young Gnome/Pixie/Sprite/Imp/Satyr/Halfling must prank the group to be accepted by or meet the dare of a peer group and the characters may not realize it is a classic scenario of peer pressure.
A magic ring is found that offers complete immunity to an element, but any party member within a certain range experiences temporary random alterations in their own items.
A water elemental demands the party drop a coin in the creek or river in order to be allowed to cross a bridge.
A prideful Orc is challenged by his friends to arm wrestle the strongest member of the group. From the looks of things they aren't giving you a choice, and they are forcing a wager.
The party was once the elite task force for the King only to find out from a Duke they were sent to kill that the King is working for dark forces, gathering the shards of a crystal which his dark master is trapped in. They must now run as outlaws and try to stop this evil being from rising again.
A Wyvern has made the city gatehouse its home. A siege is on the horizon and the gatehouse must be operational as soon as possible!
A skilled warrior joins forces with the party, but a recurring memory disorder leads her to wake up one day believing she has been captured.
Distinct armour recently looted by the party turns out to have once belonged to the father of a local hero, who identifies it and confronts the party.
A powerful arachnomancer attempts to conquer the area with his spider minions; the party is sent to kill him.
A cabal of powerful Golem-builders has sent a messenger to a Kingdom with a threat; submit to their demands or they will unleash their collosi upon the Kingdom.
Entering a flooded cave in a boat, the party members find themselves in the company of a Paladin and a Warlock. The situation soon begins to get out of hand.
A Hydra attacks the group.
Each locked in a small cell with a single tiny window, fed the barest amounts of bread and water and deprived of their equipment, the party is held captive.
The party must travel the globe searching for the pieces of an ancient set of armour.
Soon after looking into an enchanted mirror, the characters meet shadowy copies of themselves.
The person the group was sent to meet has been replaced by a doppelganger.
A powerful Demon offers each of the party members their greatest wish. This is too good to be true.
An Sorcerer seeks to uncover a massive ancient construct with world breaking power. The party is sent by the King to stop him. At the end the group must choose whether they should destroy the construction or hand it over to the King.
The chosen Queen of (race or subtype) has arrived! (May have protection and mind-control spells, etc.) She has more followers than one might expect for such a newly declared monarch. All her kin gather to serve her and reclaim their rightful place in the world.
Ogres have kidnapped the major's daughter!
A Witch had turned the village children into scarecrows.
While exploring an old rundown castle, the group accidentally releases an ancient evil and, in the process, each member is possessed by a Demon. While trying to find a way to stop the creature they released from destroying all they know, they must each wage their own personal and internal war with the Demon whom possessed them to prevent the loss of their soul.
An ancient horror (a 20th level half-dragon vampire sorcerer) has awakened...
A new Outer Plane has been discovered, that of the bleak, dead wastes of Scriothia.
The group is thrown into a game where they must find weapons and armor. They start with just the average clothing and no supplies. But they aren't the only ones in the game. Happy hunting!
The party is sent to a fort to help defend it from (monster of choice).
A terrible ogre has been treated with sedatives by the village shaman. But the drugs ran out and now the ogre is suffering from withdrawal. The party must provide him with counselling and find ways to treat his anxiety and dizziness before he smashes everyone in the village.