I disagree. A hypothetical expansion pack would add a select number of new items or maps to the base gameplay. A new game, while it's still CALL OF DUTY -- like football is football, will have many subtle differences that are easy to dismiss if you aren't a hardcore player. For example, one of the many things that switched my style up between Black Ops and MW3 was the fact that they took away the "quickdraw" ability from Sleight of Hand Pro, and made it its own perk. Sounds negligible, but now I hipfire probably 30% more than I did in Black Ops.
I've changed my approach in every COD. Differences in the map designs are ridiculously obvious to me, and I always go through a period where I test everything out to find out what works best. I'd say it takes me 4 months to really start peaking at each COD.
I really hope they don't go insane and turn COD in to Battlefield's twin brother because everybody that DOESN'T play it still complains for some reason. Uh, go get another game.
