An easy way for deciding the leaders of civs is to look for a civ that lived around that time in that region(if that nation exsisted in real timeline even better.) and make them leader.
So for the Byzantines' either John VIII, Constantine XI(Famous for being the last Byzantine in real life.) or Manuel II(Who ruled at exactly 1400 if that helps.)
The Romans are trickier since they are now a Republic however i think a good title for leader should involve Ceasar in some way(Either as a title showing that the name still is a title used by Romans.) or as a surname to represent this rulers relation to Julius and Augustus.
The Yuan Dynasty should have some named perhaps related to Ghengis or Kublai Khan?
I'll try thinking of a couple more.
So for the Byzantines' either John VIII, Constantine XI(Famous for being the last Byzantine in real life.) or Manuel II(Who ruled at exactly 1400 if that helps.)
The Romans are trickier since they are now a Republic however i think a good title for leader should involve Ceasar in some way(Either as a title showing that the name still is a title used by Romans.) or as a surname to represent this rulers relation to Julius and Augustus.
The Yuan Dynasty should have some named perhaps related to Ghengis or Kublai Khan?
I'll try thinking of a couple more.