I concur with most of the reasoning above. Korea just doesn't have the resonance to sell well; most of the others don't feature that are reasonably equal in the types of materiel they can field. There is certainly room for games that deal with those topics, but they probably won't be much like CoH.
I think the desert war is a bit of a mixed prospect. Sure it would be interesting, but I'm not really sure it is that tactical, given the lack of terrain features, and even if that could be solved, the units involved would be too early to reasonably be put up against the forces in 'main' CoH2. You'd either have to make these earlier units perform artificially well, or you'd have to split them into a separate scenario, which also splits the player base.
A while back I read that Relic had intended to produce a CoH expansion in Italy, before it was decided to go ahead with CoH2. I would like to see that, but to make a proper expansion would probably require at least one more Axis army, and I struggle to think how this could be done. If it doesn't have an army, then it would be a campaign, which is fine, and some maps, which is great, but it won't materially effect the existing multiplayer scene.
If Relic were to ask me, personally, what kind of game I would like them to make next, I'd suggest something akin to
Armageddon Empires. This is a fairly obscure, indie post apocalyptic strategy game that is nevertheless very innovative and well designed. The great thing about the post-apoc genre is that you can plausibly justify just about any mix of politics, systems and technologies that takes your fancy, and yet still have it grounded in reality. However, no fictional setting will never be quite as grabby as real world event.