These all mods will work if you have the Kingdoms expansion, if you dont have it, than the original medieval 2 mods are there too, but they are mostly abandoned, and not as good the these ones.
Third age total war (TATW for short) - full conversion mod, that brings you to the lands of LOTR, really well done and polished, has custom sttlements, events, great submods, etc, etc.
Stainless steel (SS) - Stays true to the vanilla map and expands it slightly eastward, has new factions, better AI, new skins and loads of other small things. It is however confusing to install for many and has some almost nessecarry fixes you should apply. Lots of submods.
Broken Crescent (BC) - Moves the map eastward, It goes from Bosporus to India in the east and Russian steppes in the north, it is very historically accurate too but not a huge choice in submods.
All three of these mods have the G5 RBAI available to be installed(2 of them come with it already installed IIRC, but its best to update it, as the older vesrions have CTD issues) which makes the battle AI much more smarter, they all have their strat map AI improvements as well which make allainces more reliable, etc etc.
Other good mods are: Chivalry II, Call of warhammer, Deus Lo Vult. There are lots more of them:
http://www.twcenter.net/forums/forum...-Modifications