A 3rd dropdown idea could be "Focus" or "Category" or something like that. So, you'd have (these are in DE):
Empires Ascendant (only base game civs)
New Civilizations (only those civilizations added by applied mods)
Elephant Trainers
Horse Lords
Seafarers
Settled Civilizations (this one excludes all Nomads)
This is a "nice to have" item only. All the other things from my other post are more important.