Separate app for me, and yes, I would buy it. If its a separate app, they are more inclined to re-do graphics. Furthermore, there would be a lot of new weapons, environments, and even zombies to model, meaning it would increase the size of the original game a lot. I'd say keep each game WAW and BO separate. And add Dead Ops Arcade as a separate app too, that way they could make it universal and mess with it a lot. It uses a completely different engine (I think), so once again, keep each engine different. Finally, I'd like to see some iPhone/iPod/iPad specific maps too, and above all, they need to add quick revive for solo.
Currently Playing: Hybrid (The first one), Block Fortress
Waiting For: MC5
Hoping For: RCT!!!!!