Some ideas, maybe someone likes them. in most cases (except Diadochi), lot's of native material can be used.
Pirates of the Baltic
~1390AD, around the baltic sea. Lot's of fun conlicts, and different nations. Denmark and Sweden (actually, Mecklemburg-Sweden) is at war. Sweden is helped by vitalienbrüder (privateer/pirate) blockade runners, who attack Hansa League trade ships as well. Novgorod and Poland doesn't really like the Teutonic order in the area. So, cool scenario in my opinion.
Khazaria rules:
Map centered around khazars, around 900 AD. In a very small area, you have a settled nomad kingdom (khazaria) and byzatines fighting the arab conquering armies. As a nice touch, a bit to the north you have early russians with norse influences (they borrowed the first varangian guard to Byz around this time), and also a not-yet-settled true nomad tribe (magyars)
Sicily
~1000-1100. Interesting period, as in a small area, you have the retreating Byzantines (with Harald Hardrada commanding their varangian guard!), the normans who recently arrived as mercenaries, and built their own kingdom, remnants of Benevento/Salerno (lombards I think), and of course arab taifas as well.
Norman conquest of England
It is very surprising that there is no such mod. Start the day after (or on) the BAttle of Hastings. English have big territory, but lot's of their lords and armies are lost at HAstings. Normans have lot's of armies, but cannot support them, also vassals are hard to keep in check. As added fun, Konan de Rennes (Britanny) invades Normandy. (historically he was poisoned on the orders of Willie the conq). Also, welsh and scottish are trying to figure out what to do in this case? Take revenge on the anglosaxons, or help them against normans?
I really like this idea for two reasons: factions with different playstyles, but also: norman medium (mail) cavalry with shields and javelins! Finally, heavy(ish) cavalry with no lances!
Latin Empire
~1210(ish). After the sack of Constaninople, the new Latin Empire (~western style crusader knights) is facing: the bulgarian empire, the Byzantine remnants (Epirus, Nicea, Trebizond), while the Anatolia turks, Venice and Genoa wait for a time to strike. Sounds fun.
Diadochi
Also, this would probably be the best mod ever, but would require lot's of ancient scenes.
Take a crusader map, and set the time a bit after Alexander's death. You have the Antigonid kingdoms (Antigonus, Casander, Lysimachus, Ptolemy, or Macedon/Thrace/Egypt/Seleucia in Total War terms), but also Epirus, Armenia, Ponthus, Magna Graecia. For added fun, the celtic tribes can invade from the north, the romans from the west, and the iranians (parthians, after they take Parni from Seleucids) from the east. Good times!