How about adding in a new era AFTER the modern era: like a future era? And maybe you could colonize distant planets and stuff. And then the space race victory could be not just reaching Alpha Centauri, but maybe developing the first faster than light engine or something.
Also, it would make sense to allow big enough cities to build multiple things at once, and maybe even assist other cities in construction.
If you extend the game into the future, it diminishes the scope of the game that is actually within the confines of history (i.e. Ancient --> Modern). If you add heaps and heaps of Future Era things, it will mean that less importance is placed on the other eras, which is what the game is really about. A recreation of history, not a simulation of the future. Sure, it's fine for a mod, or even another new game, but not as Civ V. It would unnecessarily diminish what should be in the game.