What do we want for the expansion...
Obviously I can't speak for anyone else, but I'd like to see a racially integrated faction (the fantastic racism/always chaotic evil in Elemental, while understandably a token of fantasy, is still something at least one kingdom/empire should be able to overcome).
Some more sovereign/unit design basics would be nice (more facial structure archetypes and skins, please)
Oh, and bigger/more complex maps! I have fond, fond memories of the announcement of 'Huge' and 'Immense' maps in Galactic Civilizations 2. As a single-player mode adherant, more content to explore is good.
The possibility for new and expanded building/research trees is of course immense... Not that there's anything missing now, but you could go -crazy- with the High Magic of the setting and the boosted level cap inherent in expansions. I'd recommend creature stables, or royal menageries, or the like, to allow those with the 'tame land' spell to get their gotta catch 'em all on. This would help highlight the ability of Empire and Kingdom alike to adapt to and control the environment, and give players who like the idea of a beastmaster Sovereign (me for one, I still have fond memories of my twin Ebenwolves) an outlet.
There's a topic on here about the Dynasty system, too, that I think you guys should look into. Heading a royal family, and dealing with the dangers and rewards of hereditary leadership/powers, is cool.
Another possible point of interest for the expansion, related to the Dynasty system, would be a system for continuing your Kingdom/Empire after the death of your Sovereign (perhaps make it an optional gameplay mechanic, since not everyone will want to go there, have it involve heavy Arcane Research) You could create a Legacy system, detailing the ascension of a new Sovereign from the ranks of your prior one's children, and how the magic and reputation of the previous/original would alter them. Sort of a multigenerational karma meter with associated rewards. A fierce and terrible warrior progenitor, dying, could transfer some of that fury and power to their heir... Likewise, a noble and wise mage would instill some of that in the next generation.
Maybe that breaks with the theme, I don't know. It appeals to me... Suppose the trick would be making it balanced, so people aren't encouraged to just get their Sovereigns killed the minute their firstborn is of age.
Another thought, and, sorry, this is probably getting into tl;dr territory... But it might be nice to have some dungeon crawling.
