There is not enough historical/gameplay weight in elements throughout the game that should be moddable:
1. Make cities moddable - cities are all alike, they need to be able to be modded to have unique buildings, either buildable only in a specific city like Constantinople or non-destructible in a city so the importance of taking certain cities is markedly and memorably higher than others.
2. Make cultures moddable - faction differences are not pronounced, muslim factions need to be able to have multiple spouses, be able to have scripted events like christians having heresy based rebellions and unrest different from the paganism spreading, heresies should be moddable to be an influence bridge between factions that is between religion and culture. Culture should affect combat given climate conditions.
3. Make armies moddable - generals, even level 15 dont survive past a few battles, and units dont matter, an army lead by a talented leader should be able to survive in a somewhat good shape after ten and more battles, so unit experience begins to matter and you get attached to you army, general. Even more so for crusade moddability, multiple factions and generals should be able to join a crusade, or at least bolster it with money and units so the crusade is more impactful to the game as a whole, same for jihads.
4. Make learning moddable - Marshals should only be able to progress in their leveling up only to a certain point with books, after which the system from KoH1 should apply, allow modders to add skills modifying temporarily the map like making temporary camps, harassment/delay fortifications, bridges. Knights should be able to be moddable to gain skills based on successes/failures be it marshals improving an archery skill when winning many battles leading archers or clerics fulfilling missions.
5. Make navigation moddable - mountains have to be able to slow armies, seasons need to affect armies, a modder should be able to restrict an army building ships to only shipyards to strengthen chokepoints like Gibraltar or the Bosphorus and to only building ships outside harbors if the general has the navigation skill at a certain level, a modder should be able to enforce river crossing penalties to armies in both battles around rivers and movement slowdown to enforce natural river boundaries.
6. Make weather moddable - both cyclical seasonal weather, scriptable weather events(diseases?) and regional weather should be able to be modded to affect building outputs (eg. farms in Sweden), armies and battles.
