I would pay good money for a Project Natal head tracking upgrade DLC once natal hits the shelves (it'd need some kind of fiddle factor though so the 'pilot' could look behind while I could still look at the screen).
I personally don't like too much MP stuff as DLC as it limits the number of games you can join if you don't have it.
Plane packs would be an obvious choice and they could be a compulsory download but you can only use the planes if you pay (like with the knothole island DLC for Fable2, if you are co-oping with a friend you can go there but you cant do any of the quests) if so F6 Hellcat please
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I'm really liking the sound of the Strike MP mode so maybe some variants or that?
Extra missions are always welcome too.