I think the best way to implement a future age and appease both the "Civ is a historical game" and the "I want to settle underwater, kill with lasers, and be 100x more destructive" camps is to make a 5th age optional in the gameplay. Just as you can turn certain options on and off before playing (i.e. certain victory conditions, accelerated production, AI respawning, etc.), I think that making this 5th age optional would be very viable. Of course, this is a moot point for C3C considering the stage of development it is at, but for a Civ 4, I think it would be great.