Tough luck With Jefferson Mad. But it was great the way you just carried on and came second in everything
Re. vassals for Askia how about you can have them if you want, but nobody else can. You should do everything you can to break up vassals/masters throughout (loose rule), any DOWs must always be against a vassal master if there is one you can reach (firm rule), and there must be no vassals except yours, at the end (firm rule). I imagine that would make it interesting for Askia not to be able to always pick on the weaklings first.
Somebody probably already suggested it but you could restrict yourself to one unit per tile for some extra Civ 5 flavour. Maybe that's just silly though.