I have two Ideas I believe would be interesting mods. I'll begin with the first most obvious mod idea to never have had a mod, that I know of.
Charlemagne Total War, or Karl der Goße Total War, or Charles the Great Total War. Depending on you nationality and cultural preference.
For those who need a little brush up on their Dark Age/Medieval history this guy was King of the Franks from 768 - 814 and was the first Emperor of the Romans, or more commonly called Holy Roman Emperor. Both the Kingdoms of Germany, and France as well as the defunkt Holy Roman Empire claim him as the Father of their Kingdoms. He is labeled Charles I in France and Karl I in Germany/HRE. Through marriage alliances pretty much all Western and some Eastern European Royalty share a blood line with this man. Oh yea he stopped the Moors advance into Europe. http://en.wikipedia.org/wiki/File:Fr..._to_814-en.svg