I'll still be playing Modern Warfare. I will see how Black Ops Cold War is like, and if I like what I see, I will play it. My big issue with Black Ops has always been the theater. I tolerated WWII, but it was because of the actual game itself. I was a big fan of the "War" mode. I like the controls, and the general gameplay apparatus. My big issue is all of it is in World War II, which stifles creativity. You are stuck with the era's weapons, attachments, style, etc. Unless you can merge Gunsmith into Black Ops Cold War, which would open up a lot of opportunities. The issue is the base weapons will be so limited. You'll be stuck with 20 weapons, and CODMW had at least 50 by the end of Season 5.
Modern Warfare set up the roadmap, as far as the perks, killstreaks go... but weapons? It's going to be limited. Always has been, and always will be. Treyarch will have to do more than just improving the game engine, they will have to improve EVERYTHING. Expand EVERYTHING. Otherwise, this game will only target the "history buffs" of the community.