Halo 4 will get to 8 million no matter what. Halo Reach (although not a proper sequel) is around 9 million. I do think the game has to be great to get past 10 million. It's a different game maker, so it could go either way with the quality of the game. Going back and playing Halo 3 and Halo Reach (although a little different), I don't think the multiplayer is as good as either Battlefield and (gasp) COD. Not sure if the multiplayer will have longevity as other FPSs.