Can only speak for myself but as long as COH has WW2 background we will always have the same units and thats no problem for me (Cold war would be my favorite for COH3 because of the fresh setting) as long as RELIC would provide modding support, better ingame communication options, better replay options etc and CERTAINLY a better and more motivating ladder system with different seasons (like Starcraft) and so on. I would also expect to bring back some of the best maps of COH2 (Crossing in the woods with Mediterranean setting for example).
One thing that no rts can offer is a tutor system for players. It would cool if new /midlevel players could train with highranking players and the highranking guys would get more ingame currency for every game they play with the guys seeking for help.
There's also another reason for them keeping the franchise in WW2, assets. Most of the stuff from CoH2 is simply refurbished from CoH, I especially took notice of this from the British HQ truck model, it only being slightly modified. Sure there are entirely new models like the Churchill VII, Jagdtiger, Sturmtiger, Comet and so on but if you take the time to look at every single model individually and compared them to CoH you will find striking similarity.
But I agree with Crecer that probably improving upon CoH and making expansions for it would have been the best option as CoH2 is basically a rehashed version of it based on both the Eastern and Western Fronts with somewhat similiar units for the most part and shittier campaigns and graphics in my opinion.
CoH is a stable WW2 platform that could be expanded on further as mods prove but I would like for it to venture out in No Man's Land of WW1 since it's a relatively niche sector of RTS games anyhow, for those that say it would be boring because of trenches, the warfare of 1918 was much more mobile than the previous years and could be used for a basis with more automatic weapons like the MP18 and so on as The Great War mod team for CoH decided to do. Iron Harvest being based on CoH 2 years after WW1 ended but with the added twist of mechs also generates a lot of interest.
But your idea of a tutor system for future games does indeed sound nice, only thign is for them to execute it well.
With all of that said, I'm still hoping for Relic to make the Eastern Front mod into an official expansion as it's basically that right now but not officially and an Italian Front expansion, just add official modding tools and mod workshop support (not just for maps) and you've made a WW2 game that covers a very good portion of the war and an even more stable and solid foundation for a game.