Idea for civs from different eras.

I have been thinking of a way to add more modern civs while maintaining balance.

By looking at history, certain civs spawned others or they have continued into present day.
If you use the levels to create the next age set, then perhaps once you get to level 40, you can get a choice of relevant civs. This will obviously be less for AoE3 eras.

For example, romans at lvl 40 can become French, Italian, Byzantine, Spanish or English. Celts can become English, Prussian, Slav or Scandinavian.

You could also make it that you get handicapped if you play with a more advanced civ against a less advanced one (like America against romans). This might be better as China, Greece and Italy have technically been around since ancient times. Also, Rome was originally a Greek colony.