Byzantine empire, should not be able to unconditionally surrender. And actions on Constantinople should never be option in peace deal if not directly captured.
So, after playing Balkan nations for a few runs now, I noticed one constant: the Byzantine Empire tends to lose Constantinople quite quickly after the start date, often in the first 2 wars.
In the first war with Bulgaria, Serbia, or the Ottomans, AI will take Constantinople, or, even worse, make Byzantium destroy the Theodosian walls around it, making it easy for anyone who attacks the state afterward.
Of all unimmersive things in EU5, for me this may be the biggest one, and it does have bad effects on the gameplay of the Balkan region as a whole, because it more or less becomes a race for Constantinople in the first 2 decades of game time, while eliminating Byzantines as potential players entirely by the 1350s.
As I said, besides this making very little historical sense, this makes the initial decades quite boring and repetitive: as any Balkan or Anatolian state, Constantinople is the largest market in the region, and a city with a large population will boost a state's economy quite a lot, especially in the early game, wasting cabinet actions to stabilize it, assimilate it and integrate it, is 100% worth it.
Taking Constantinople is the best tool to avoid the struggle of the early game economy, and this makes it a prime target for META plays.
This eliminates quite a lot of strategic decisions in the early game, and opens insane snowball effects later down the line.
I do think Constantinople, and by extension the Byzantine Empire, should have special war mechanics unique only to them to mimic the defiant nature of the last Roman city and its citizens, as well as the importance of the city to Orthodox Christians
The Byzantine Empire can not unconditionally surrender, nor can any action be taken on Constantinople if the city is not directly taken.
When Constantinople is besieged, the Byzantine Empire can call Orthodox nations to aid by using prestige and legitimacy.
Byzantine Empire can be vassilized but not annexed, nor can Constantinople be taken from them with vassal actions.
Byzantium can commit its manpower to the Constantinople garrison.
During a siege, if Constantinople is owned by Byzantium the can launch a counterattack action that would cost some defenders but would reduce siege progress and do damage to the attacker's army.
In case Constantinople is taken, the Byzantine Empire collapses; Constantinople becomes a core of the state that captured it instantly; however, this causes massive local antagonism and persistent antagonism until the end of the game in Orthodox states if the owner is not Orthodox.
These changes can reintroduce a lot of historical immersion to Constantinople and the region in general.
Firstly, it makes the city a lot harder and costlier to capture, and it was historically.
Ottomans, for example, would need to first eliminate and pacify Balkan states, secure their borders against Georgia and other Orthodox Eastern European states, and build a strong enough navy to secure the siege and maybe even blockade the city for a long time before committing to a siege.
If the city is captured by the Ottomans, there would be a constant threat from Orthodox states, as it was historically, with each of them aiming for eventual capture of Constantinople.