Why the limit on conquered provinces? The game counts protectorate provinces as yours, so it would be very difficult to get every AI faction to be protectorate without reaching the 50 limit.

Diplomacy "features" would also make the game very annoying. Some of your protectorates would make alliances with non-protectorate factions, meaning that if you attack those factions then your protectorates will rebel and you have to capture them again. I think it's also impossible to make all the Roman factions to protectorates unless you can somehow trigger the civil war, since they're all allied to each other and as soon as one of them becomes protectorate, the others will declare ceasefire with you (yes, declare. they don't bother to ask your opinion) and I don't think it's possible to demand a faction to be protectorate unless you're at war with them.