I'm with Theben on this one - the game could end with a successful manned space flight to Mars. To be able to launch such a project would require a few space improvements such as shuttles and an orbital production facility (or rather facilities) and perhaps a space station (maybe limit one per civ, ie like the palace/capitol) that would serve to actually assemble and launch the ship - so it would be like having just one "city" in space. Of course the station would have to be built using the orbital platforms (and shuttled parts) whilst the platforms would be built from shuttled up parts (or other existing platforms). This seems realistic enough whilst also being a significant, game-ending endeavour, with all the features of a space race. I'd axe the possibility of space warfare though (other than spy satellites) so the only way to win is to get to Mars first or to conquer the other civ on the ground. Finally, this has the advantage of limiting future techs to a few realistic near-future techs.

What do you think?