a massively multiplayer online zelda-esque 2D top down Action RPG with all of the modern RPG elements like crafting, detailed skill tree's, auction houses etc.
There would be towns that would get invaded by monsters and you'd have to drive them off and hold them as a community or they would get over run (even permanently)
Some quests would be created any time by fellow players and can be picked up by any other player like a bounty board.
Dynamic world events that everyone can take part in.
Vehicles!
and amazing sprite graphics like any of the old classics