you know what, in my opinion, would be cool ..
think, lego starwars, lego indiana jones, lego batman, etc ...
that style of gameplay. make it an MMO. make it so you can level up. make studs a currency. have unlockable characters or classes or something .. have lego city as a hub, have a big world with lego mobs in it .. and then dungeons would be levels from the games.
just an idea ...