Louis XXIV said:
I'd suggest expanding the Space Race to include other goals (such as first in space and first to the moon). First one to complete this goal gets cultural and scientific (free tech?) boosts. Those who complete it later also get advantages, just not as good. These smaller steps would be needed for a space-race victory, but could have other uses as well (the free techs could be military techs instead, reflecting the practical applications of the space race and offering alternative reasons to persue production in this area. I think this idea will add variety that isn't necessarily military variety.
I like the idea of changing the space win to more of bonuses or wonders such as the circumnaviating(sp?) the world, where maybe instead of +1 (or whatever it is) movement it give sbonus to science or what not. However, I believe that then you would need to stretch the tech line to make it actual worthwhile, how useful is a +50% beaker bonus (just an example) when the game will be over in 50 turns anyway? The changing of the apollo program into a couple of wonders or bonuses is a great idea that I think could be added as a mod, with the adition of more technologies to extend the tech tree (for example extend it into the "near future" with robots like Mechwarrior's etc).
However, I still sustain one of my original points, the problem with this is that the game has got to end at one point... even if it is not won with the space race, a lot if not most of the time you know who is going to win. The amount of time that EACH win takes would have to be increased (even domination or conquest, maps would need to be made bigger etc) in order to have the modern era techs and any techs that come after a chance to make an impact in the game. This is the reason why I don't see it coming to Civ IV, and perhaps in Civ V where they (I am assuming here) will take a look at what worked and what didn't, and then try and make aditions to that (so if they wanted the modern era to have more of an impact, make each victory have to take longer as I mentioned).
In a game, such as the Sims, where the game virtually doesn't end, these ideas would be great, but would need to be followed by an eventual end to the tech tree (you can only be so creative).
This brings me to another point, if the game were "everlasting", then a system would have to be developed in order to "introduce" new civs somewhat developed. Maybe something like some people mentioned, where one (or a few) of the cities in an empire revolt and gain units to try to gain independence (whether it be maybe by a number of turns it must hold out or something), as well as recieve part of the techs that the original civ holds. This can introduce a whole new strategy, where as some of the cities of your enemy revolt and gain independence, you help them out with units (or declaring war againts the civ yourself), or what not, in order to damage your enemy and gain a new ally (think US independence and france).
However, for ANY of this to happen, the first step would be to add more content to EACH age, and make EACH win type last longer than they currently do (including making maps bigger etc).