I'm not sure what will happen, MW2 has far surpassed the sales on Halo 3 just in pre-orders. It spans all 3 major platforms: ps3, 360, and pc, unlike halo 3 which is the abusive husband of 360 who still pays the bills. MW2 couldn't possibly live up to such a horrendous amount of hype, it will be an excellent game nonetheless.
What you're getting at seems difficult to comprehend though, seeming as Cod4 went toe in toe with halo 3... ODST isn't nearly as large in scale of grandeur (or so I've heard) as Halo 3 to be comparable to MW2 though, so maybe halo 4...?
What you're getting at seems difficult to comprehend though, seeming as Cod4 went toe in toe with halo 3... ODST isn't nearly as large in scale of grandeur (or so I've heard) as Halo 3 to be comparable to MW2 though, so maybe halo 4...?