The Crystal Dragon on the Mountaintop
An ancient Crystal Dragon wyrm lives on the top of a mighty mountain peak and has subdued several white dragons to his icy will. His thirst for knowledge of the world and everything in it is surpassed only by his laziness; he sends out his polymorphed white dragon minions amongst the ordinary people, so that they can abduct the wisest and most knowledgeable humans and bring them to him for "conversation". The most entertaining he keeps with him; the rest he sets free - although they have to make their own way home from the mountaintop.
Rumour has it that the wyrm's knowledge of the world is now so great that he can answer any question he is asked, like an oracle. Of course, there is no guarantee that the answer he gives will be the correct one. That all depends on his mood...
The Parsimonious Emerald Dragon
The players find themselves in the clutches of an Emerald Dragon who has been the victim of too many adventurer-thieves over the course of the centuries. Now it is the last straw; the players are going to track down every item of treasure that has ever been stolen from the beast, down to the very last copper, the very last piece of opal. And the dragon is going to make sure of it by dominating them with mighty geas spells which will drain the life from them if they disobey.
The Topaz Dragon and the Crabmen
On an inaccessible stretch of coastline of craggy rocks and sheer, high cliffs, live a tribe of ignorant Crabmen. They have begun worshipping a local Topaz Dragon who has taken to sunning himself on a patch of rocks near where they live; every few days they bring him some of their babies as sacrifices (they don't mind; they give birth to thousands every year) and in return he goes and attacks the local Sahuagin every so often with his psionics and cones of dehydration.
Then some innocent fishermen - or perhaps not-so-innocent adventurers - happen to kill and eat some Crabmen, and this makes the Topaz Dragon very, very angry. Not because he likes the Crabmen. But if humans go around eating them it means less baby crabs for him to eat...
Too bad I can't use all of them; I think my players would get dragon-ed out if I did.