The Last Crusade :
- new mechanics for crusades and holy wars (disputed territories, mechanics for when big armies cross your territory and destroy everything, forced walk...)
- new dynastic mechanics, with cadet branches
- heraldry overhaul (with proper colours, more variety, and meaningful changes, with
cadencies at least ofr bastard, and fusioned coats of arms for dynasties who... well, fusion)
- new navy mechanics (not necessarily proper fleet combat, but building a fleet has been great business in the Middle Ages, it should be more complicated than it is currently, including for Vikings - you should need to find the wood)
- cathedrals (long term building, very costly...)
- monasteries, or rather monastic orders (that works kinda like a hybrid of merchant republics and military orders, and more interesting than purely theorcratic governments ; can also be adapted for Indian monastic schools)