Personally I've been thinking about the idea of a Medieval 3: Total War. I think that it would be the most entertaining, and with today's gaming engines they can vastly improve the game. For example more intricate and better interactions between the soldiers when they fight. It could also be a goodopportunity to improve on details from Medieval 2, for example introduciing naval battles, improving soldiers, siege equipment, campaign map, the option to hold prisoners, diplomacy, etc... The list is almost endless.