I thought there were some units (e.g. the Winged Swordsmen) that aren't available in the campaign but only in custom battles? Are you planning on reintegrating these? If not they could be replaced by other units of Taro.
Regardless of that I have to say that maybe some other faction rosters should get some attention. While Gondor is probably the most popular faction and their fiefdom troops add a lot to the immersion, their roster is already many times bigger than some other factions', e.g. OotMM or Dunland: the custom battle rosters show 44 Gondor units against 15 OotMM and 16 Dunland units. Gondor also has more volunteer units.
Some units are even kind of redundant, like Dol Amroth's siege engines whose only difference to the standard ones are the looks of the crew.
So I think there is definitely room for other units, although I think it shouldn't be Gondor ones. On the other hand: Are there even good unit submods for the evil factions that haven't already been implemented?