Hey so I had this idea. No idea if it's any good. But let's see.

So here's the premise,
We've got this huge magical fantasy world. Full of wizards and dragons and all that good stuff.
But we're not going to focus on that. A lot of spell casters and magical beings use spells that conjure familiars to do battle or perfomr labor. From a wizard's familiar, to a druids summoned truant to a paladins mount. This world is full of magical ways to get some temporary aid. Question is: where do they go when the spell ends? Where are they when they are not needed?

Well most go home to their native plane, but there are some who know of a place.
A place located on the edge of the astral sea. Where they serve the best drinks this side of the multiverse.

The game will revolve around the Familiar Tavern.
You'll play the summoned creatures that frequent it, in between their duties to their mortal caster.
Pretty much anything goes as long as it's something that can be summoned in a fantasy game.