You know, when I played Black Ops when it came out, I was positive that it would bomb. I said to myself, "wow, they really didn't try at all with this one, this is going to create a rage storm."
But no, everyone loves Black Ops. So really, I'm positive they can do whatever they want, as long as there are a few new perks and text still pops up in your face to constantly remind you how great you are -- then CoD will always sell.
They could screw it up very easily though, I bet adding recoil would piss a lot of people off, for example.