I would like to see them expand on WW2 with the Eastern Front, North Africa and the Pacific. Whether these should just be more expansions or part of a sequel I don't know. I've seen some people suggest Vietnam and I would love to play a Vietnam-era RTS (the closest I've got to that is a downloaded scenario for Empire Earth). However, I have also seen people say it wouldn't work as it's own game. So maybe CoH 2 could be about Cold War-era conflicts, such as Korea and the Afghanistan War, with Vietnam as an expansion. Then I've also seen modern warfare suggested. Again, a great idea which hasn't really been covered in the field of RTS. But I'd like to see real events represented, instead of fictional conflicts such as in BF2, CoD4 or ArmA. The Falklands, Iraq, Somalia, Afghanistan. Afghanistan could be an expansion with the Coalition vs. Taliban and for the other three, the campaigns could be British Military vs. Argentinian Army, US Military vs. Iraqi Army and Task Force Ranger vs. Habr Gidr.
Hmm, my favorites would be either Eastern Front or a fictional Cold War goes hot scenario.
In COH 2 I would to see aircraft buildable and you are able to watch dog fights up in the sky while you are fighting on the ground, Now that would give coh 2 the feel that like a whole new game.