Been playing a lot of Sword of the Stars lately and also talking to Sidious about Galactic Civilizations II and it has identified in my brain two titles that I would like to see made:
Alpha Centauri II - Dude, Alpha Centauri was sweet, it had a bunch of cool future techs, a neat sci-fi "plotline," and the ability to design your own units. Just fantastic.
Imperium Galactica III - I loved the way the Imperium Galactica games combined the RTS and 4X genres AND tried to have interesting stories at the same time. Imperium Galactica II actually had different storylines for each of the different sides (there were 3). That was a great series I'd like to see more of.