Games you'd buy in a heartbeat if they existed.

[Myviewing]

Sonic Adventure 3
Drakengard 3
Any Pokemon game
Castlevania: Ressurection (Was a DC game, got canceled.)

But the one that I really would do so: Legend of Dragoon 2
If I learned this game was actually in development, I'd buy a PS3 JUST for that.

But in my wildest unrealistic fantasies that wouldn't happen in humanities lifetime: A jointly made game made by Nintendo, Square-Enix, Sega, Namco-Bandai, and Konami. Existing Nintendo franchise, maybe. New IP would be more preferable though.