I'm not sure. I don't think that there is much CoD can do anymore for me outside of hit the reset button. The games have become saturated with content, but none of that content has added anything to the series since MW2. Well, I guess Advanced Warfare did have the power armor, but I found it so poorly implemented that the game probably would have been better without. Really, hitting the reset button, going back to basics, and maybe changing the system entirely could help them recapture what made CoD4 so great: a simple enough game that was more concerned on offering the best experience with what it had than with drowning the player in 50 different guns that only offer 12 real options.
As for singleplayer: Don't care. It stopped being relevant in CoD around CoD4 (minus Treyarch's attempt in BO2 I guess). Actually, even back at the original CoD, and especially by CoD2, it was easy to see that the series would probably start going down a multiplayer route sooner or later. And honestly, I don't feel like trying to recapture something that lost relevance almost ten years ago, especially when the fans have clearly pushed it towards a multiplayer-focused series. Actually, I think I'm sort of surprised it has stuck around this long. I would have thought that at least one of the developers would have tried just scrapping it and using the extra resources to add to the co-op.