Imagine Oblivion only pirate based, with Gmod's ability to create and customize your own boat after having to collect (or purchasing) your own supplies. Then multiplayer/lan so that you and your pals could go together on your boat and do stuff. Then, if you felt like it, you could go on land, depending if there are people (AI) or not, you could pillage, and then you could terraform the land however you want it (Minecraft only more detail), build structures, like Blockland. (only with more detail like Gmod, but a basic easy grid functionality.) There wouldn't be a set story line that you were forced to play to gain anything, but there could be side missions or tasks you could do. (Oblivion minus the main story line) There would also need to be the ability to change your beginning, like how you came to be in the game. (This was the only turn off to oblivion was that you always started out from the jail and leaving the sewers.)
Tell me you wouldn't play the stuff out of a game like that.