A thought on changing leaders vs changing civs
Design wise, aside from how consistent having ability A is on leader B vs civ C, there's no reason why the roles of leaders and civs couldn't be reversed, leaving the mechanics unchanged.
That is, using the notation [leader/civ] (X) meaning this leader/civ has ability X, instead of say Montezuma (A) going through Aztecs (B) -> Songhai (C) -> France (D)
We could have Aztecs (A) going through Montezuma (B) -> Benjamin Franklin (C) -> Karl Marx (D)
While some combinations might be weird, I don't think that's a good argument either way.
Currently I don't see a reason why the roles couldn't be reversed, but I'm happy to be proven wrong