As soon as I saw Double Fine's Kickstarter, I thought to myself, "I'd love Obsidian to do this for an RPG featuring turn-based tactical combat and a top down perspective."
I'd prefer a true turn based game like Fallout 1 & 2 (as opposed to Baldur's gate where the "turns" were mostly behind the scenes). And In combat, I'd like to have full control over my party members' actions. Oh, and a party of 4-6 (including the PC), as opposed to just one companion (which seems to be the norm these days...)
I'd suggest an original IP. Something sort of "weird steampunk" (a la Planescape: Torment).
I also like meaningful choices that change how the story plays out. I like my companions to be integral to the story (a la the Sith Lords). I liked how Torment was philosophically interesting. And I like factions... especially when some factions are mutually exclusive (like New Vegas). I really enjoy a bit of complex political interplay.
Honestly though, if Obsidian put up a Kickstarter and just said "we'd like to do an RPG our way" I'd totally fund you guys, since you guys are one of the few studios who consistently make the kind of thing I like.