I doubt we will see a SMAC2 for a long time, if ever. First off, I get the impression, (I may be wrong) that SMAC was primarily developed by the folks who left for Big Huge Games, with Sid Mier only `advising' them. So I'm not sure if Firaxis will ever see fit to redevelop the game from the ground up.
But if they DID, I can think of some stuff I'd like to see. First off: Different Planets. In other words, you don't always land on the planet of SMAC with pink fungus and trancendence and stuff. Sometime it's an Earthlike planet. Sometimes it has a pre-existing civilization (in this case the humans might remain united, or almost so). Sometimes it's like Mars and terraforming involves the creation of oceans and a breathable atmosphere. Etc etc. I think there are alot of various types of planets you could play on. Of course, you could either pick the style of planet, or have it randomly selected.
The other fun thing SMAC2 would need is space travel and multiple maps. This way if you came into the system and found the planet with a pre-existing civilization, you could settle on ANOTHER planet in the Alpha Centauri system. This would also allow the settlers to split up when they entered the system, colonizing various planets. Balances would be made so that the folks who settled the biologicaly rich world might have less minerals then the folks who settled the dead one.
This would also facilitate the possibility of having more factions playing at once and civil wars etc. Children of Planet split off from Gaian's in bloody civil war and so forth. Other spaceships (better equipt) could be launched from Earth behind you. Heck, when the Earth discovers the same technology you have and you've both got interdimensional gateways and stuff, maybe Earth decides it's time to reclaim its wayward colonies and invades Alpha Centauri. (You could even fight your old Civilization from Civ3.. including sending bombers back to Earth and blowing up all the stuff you spent so long building before.
)
Of course, if they were going to do a game in this vein, I would MUCH rather they kept it in the solar system and the realms of slightly more realistic science. Then it could continue right after the end of Civ3... with civilizations building up on the various worlds of the solar system... rebelling, fighting, developing tech etc. They wouldn't all have to strictly be `factions' like SMAC either... the Lunar Free State. The Martian Alliance. Bla bla bla.
In fact, I'd rather see a neat game centered around the future development of civilization in the solar system opposed to a simple reworking of SMAC.
But maybe that's just me.
Joe
[This message has been edited by Joe Bourque (edited May 12, 2000).]