More Likely Royal Marriages Between Heretics and Heathens
In the vanilla royal marriage system, the penalty for heretics is -100 and for heathens is -200. If you play as Byzantium, you can only seek marriages among the Slavs or Trebizond—practically unwanted goods that nobody wants. This is too tragic; 14th-century Byzantium was really not that desperate.
I have changed these to -60 and -120. Marriages between heathens remain somewhat unlikely, but marriages between heretics are now perfectly viable (that said, you still need to improve relations if you are not an Empire).