You got some grasp on history and a decent plan to save AoE 4. Now that think about it, I believe the only way to revive the game is a sensational epic “Superpower Clash DLC: East Roman Empire x Ottoman Empire”, with the Crusader States in the background or Roman allies.
Question is, should Crusaders (also knights Templar, Hospitaller) be Byzantine-allied units available within the ERE civ?
When these Crusader “allies” were allowed into mighty Constantinople by political infighting and saw its amazing wealth and grandeur with envy, they turned marauder. They ransacked, burned, destroyed, looted, raped far, far worse than the relatively tolerant and disciplined Ottoman imperial army, after smashing its way with monster-size bombards.