Synergy bonus for transitioning to relevant cultures

I personally enjoy the possibility to go wild and get a unique cultures combination perfect for the flow of my particular game, so I wouldn't want it in the base game.
But I can see your proposition become a game-mode - a ruleset that can be toggled at game creation called "historic" that would have such bonuses. So that some players can play a wild choices version, and other players can play a more historically accurate one.

I'd prefer it to see if cultures that are more distinct blend in some way, because going from the Romans to the Aztecs is a huge leap. I don't know how they could do this in a way simplistic enough to implement but there's gotta be something they can do to make cultural progression feel natural without forcing people to stick to strict historical progression.