Makes me wonder what the next expansion will have.
More alien variety and gameplay.
4 more factions.
More biomes and maybe a global event/quest system.
What else?
New alien species would be cool, maybe more flying ones so that they can threaten land and sea? They can even be "unseen" ones, which infect tiles with some alien looking goo or fungus and needs to be cleaned. This could happen if you linger too long in unhealthiness -- and the deeper you go into unhealth, the less time it takes for it to start appearing on tiles. Maybe it even slowly damages units on them, like miasma.
Gameplay-wise, I'd like to see an expanded and more fleshed-out orbital layer, along with better air gameplay (it's sci-fi... you GOTTA focus on air in the next/last expansion). We can't just focus on the ground and water like a traditional Civ game.
Maybe unique new orbital resources can be space-mined and exploited for new types of units/buildings.
Maybe tacjets can take to the orbital layer and move much like a normal unit would, instead of having to be based within a city or carrier. When attacking below, you perform the swooping attack action from the skies and return to the orbital layer. But from there you can also be shot down by units which can fire at satellites.
Maybe carriers can upgrade to a hovering early starship which can carry the upgraded tacjets. They can also fly up to the orbital layer and move around there as well and protect tacjets from being shot down since it'll take a lot of missiles to shoot down a starship within the orbital layer.
Four new factions would be great. They don't need to be based on countries either; they can just be corporations, cults, etc. Basically, any kind of group that can create/buy or conquer/steal a seeding ship.
Non-playable, intelligent, and powerful alien factions could starting making planetfall midgame to spice up the later half of the game. Or instead, maybe even a couple technologically advanced human factions from another planet which have been seeded much earlier than your own can start appearing and colonizing your planet. They would always be hostile, though, since it's a pretty aggressive act -- they're strong and everybody else is weak, so they're there to conquer you. These late NPC factions could also be a random affinity combination in an advanced state -- you could be dealing with a cyborg supremacy leader, alien-like harmony one, a monstrous harmony/supremacy leader, a cold and pretentious purity/supremacy leader that looks down upon you and has all kinds of gizmos and things worn on them, a purity space admiral, etc. (Though, I still think aliens would be better, and wouldn't need to be hostile towards factions all the time, plus they wouldn't be competitors, they'd be truly neutral things to interact with.)
Psionics can be introduced and be a sort of mystical side to the game which players can develop towards. In some ways, it can perform similar to religion in Civ 5. Great people can be reintroduced, but instead, they're called Gifted Ones and are periodically discovered or born in your empires depending on the amount of psi you're accumulating, and can be used to perform great things.
Hybrid affinities can be fleshed-out more and made into their own affinities alongside the primary ones. Own title, icon, complete unit and city graphics, more unique upgrade abilities instead of copying the primary ones, maybe even their own unique victory types. Though, everything is still based off of those three primary affinity point types -- just like with colors. You don't call crayons blue/yellow or red/blue, they're green and purple.
Biome-wise, I can't really think of any other types, at the moment. I think things are covered pretty well.
Oh, and call it all Falling Skies!
