A cool idea.

I think there should be a Mod that sorts countries out in groups like Great Powers,Middle Powers and Regional Powers. The bonuses should be more gifts from city states.bigger chance to make someone your vassal etc.

How would this actually work? If a civilization got big enough would they be "promoted"?

Or are you just suggesting historically important civs get extra bonuses?

A system that counts military power,population,production output,food quanity,land area quanity and diversity of resources,coastal cities and how much city states you protect with your military. Every 10 -12 turns you get the new race like with the Culture Capitals mod.