They could make a Halo Wars2 and use Natal as the controls. Like to group units you make a circle around them and then "point" to where you want them to move to. That would be really cool.

I don't think it will be possible for Natal to be used in a FPS without another controller or accessory. Watching the Natal team play Burnout on Jimmy Fallon, you could see that you had to use large motions for simple things such stepping on the gas. Unless you have to make your hand like a gun and create "recoil" there isn't much you can do with a FPS.