I would also say that the North is unbalanced, but it is unbalanced right now imho. Gundabad gets almost always owned by the Dwarvs. They are sandwiched between Ered Luin and Erebor. At least in FROME.
So, Gundabad should get some improvement in order to survive longer against the advancing Dwarven Hordes. Rohan is very seldom facing the OoG, since Rohan has to deal with Dunland, Isengard and Mordor, but only during crusades with OoG.
Though with Il Ducces Extended Settlement Mod, Rohan is surviving much longer.
So, now I found what I was looking for. That Mod was planned by Beorn. It has already a very nice Roster for Rhudaur. So, if not all of them could be AOR Units for Angmar.
Also, Arnor could get some AOR Units from the Cardolan or Arthedain Roster. Maybe Militia first and second tier units. I do not say, give Angmar or Arnor alot of Super Units, but for sake of variation, give them more AOR low or middle class units. In that case one would build up military buildings in specific AOR settlements in order to be able to recruit this units.
Same as it was done with Gondor, by Taro M's Unit Mod. Well, of course Beorn should agree to use his units for an Arnor Mod. Unfortunatelly I have just a few ideas, but no Modding skills.
http://www.twcenter.net/forums/showthread.php?t=353006