Historically based, yes, but history shall march onAlso there are a lot of technologies/ideas that are current that aren't really covered by the tech tree now. For example cloneing/advanced genetic engeneering, terrorisim, bioweapons, modern agriculture (being able to plant resources like corn and rice rather than being dependent on the map), and invisiblity (which is actually way closer than anyone previously imagined).
I don't mean necissarily taking civ enormously far into the future, but 100 years couldn't hurt. I think it's fairly safe to say what technologies look likey (for example Fusion is already there - its an obvious next step). I also think that there need to be social techs that go into the future, not just scientific ones. About the seas and space - I don't think we are anywhere near either of those, except merely to gather resources - so I don't think space-sea settlers are an option unless it's the extreme future. BTW - Can you imagine the cost of upkeep for space cities (!!).
The whole last 1/3rd of the game is kinda screwed up in my view with units moving way too slowly and turns going way too fast (I mean does it take from 1970 to 1975 to get units across a large continent). Some technolgies like Flight are way less of a bomb than they should be. The ability to get stuff anywhere any time should be a great boon, but a lot of the time it's just a distraction to getting further up the tree.
I think Civ has moved well to the right direction with the civics system, and I would like to see more of those in the future.
I've kinda rambled but I think I got around to my major points. It's not like we are ending at some major juncture, say industralization or the World War II era, it just sort of peters out and that just leaves me (and many) feeling unsatisfied. I mean, obviously we are still going . . . so keep the game going.
P.S. Oh yeah, I think Postmodern should be an Era, and I don't think universal sufferage should be so easy to get. but thats just my 2 cents.
Now that you have said that, I think we can agree on enhancing the Modern Age (especially) as a goal! Space cities are kind of ridiculous, I think, as are underwater colonies as somebody mentioned earlier--sure, we have some underwater bases, subs, etc., but not hundreds of thousands of people beneath the waves.
I would like to see an automobiles technology that increases the movement along roads (again), and a similar tech for rails. Also, modern armor and mech infantry should move at least 3 squares, if not 4. Upping the movement of some Industrial Age units would be nice as well.
Second what you have said about Civics--that was the feature from SMAC I wanted to see in Civ3...and it wasn't there. I'm glad it got into Civ4.
And I never said the game should flat end at the Industrial or Modern era...but I disagree with things like walking battle-mechs and massive laser-shooting armies and other "scifi" style technologies. Keep it real--railguns, fusion, trimarans, maybe more kinds of aircraft (like dive bombers that can be stationed on warships, or transport helicopters that can drop off infantry), cruise missiles, tactical nukes that can be loaded onto nuclear submarines, etc. There are plenty of currently-existing technologies that can be fit into the Modern Age; we don't need to add some ill-contrived future era to the game as well when we are ignoring so much that can be implemented and still keep a historic/contemporary flavor.