The only way CoD can refresh itself is to not release until the next gen of consoles ship. A new game engine is a must (Why is an ancient id engine still being used?!) and a longer time wait to iron out bugs and glitches (sorry to the glitching community) is needed. 2 years at least. A massive revamp is needed.
As far as settings go only the future is left to look at; but care must be taken not to become too close to Halo. I've posted on this before.
Pre-WW2: would be a bit boring, unless CoD abandoned FPS and became more of an Assassin's Creed or Red Dead type game. Imagine how the loyal fans would treat that?!
WW2: Done to death and then some.
Cold War: Black Ops 1 did this.
Gulf War / Afghanistan: Too much like MW and the BF series, and MoH did it first.
MW: Like WW2, this dead horse has been flogged almost out of existence.
A Star Wars CoD would be a very interesting prospect following the CoD 4 mod.