Since the dev's won't talk. How about doing what a forum does best: speculate!
New Faction System: Join a royal faction and use your allies in the party to enhance your strength and tear down rivals
Forming factions will probably be DLC only. Joining factions may be for everone. Having factions will probably be for everyoune as Byzantines are supposed to be playable and it would be strange if the system would simply disappear if you (the player) inherited Byzantine Empire without the DLC.
Raise Standing Armies: You will now be able to use retinues to have standing armies in your domain: the size of which is determined by technology
Probably DLC only - small armies make little difference and the human player is always has an advantage anyway (of being human).
Experience Factional Revolts: No more easily defeated rebellions. Disgruntled vassals will now band together in revolt against your rule through their faction
Seems quite AI-focused, also an expansion of the firs one.
Appoint Orthodox Patriarchs: Orthodox kingdoms and empires can now control their own heads of religion and their powers, instead of being dependent on the patriarch of Constantinople
For everyone - it would be too weird to adapt the game to a situation where the rules fluctuate based on who is King of an Orthodox nation. Pentarchs may be DLC-only.
Streamlined Mobilization: You will always raise a single, larger levy from your direct vassal; no need to worry about the opinions of the lower vassals
For everyone.
Leader Focus on Combat: Appoint your generals wisely, their traits and skills are now of vital importance on the field of battle. More commander traits are now added to increase the importance of your choice of military leaders
For everyone.
Byzantium Comes Alive: New sets of decisions and events specifically designed with the Byzantine Empire in mind
If they dont make it free some's just going to mod-in similar events/decisions unless they're really unique like Hajj.
Improve your ruler: You can now actively strive to improve your skills or traits through the new Self-Improvement Ambitions
Possibly DLC-only.