3 unit suggestions
I would like to suggest 2 units.
"Half-orcs" for the faction Isengard:
They were usually the product of a union of Men and Orcs. They were not as wide spread as Uruk-hai, but they were still deadly. It is recorded that Half-orcs came to be by an evil act of sorcery where Saruman bred Dunlendings with the Uruk-Hai, the evil Half-orcs were the result of this union. The Half-orcs were slant eyed like orcs yet as tall as men.
Not sure this could be used as some kind of "medium infantry unit" or more like a "elite infantry unit" for Isengard?
"Goblin-men" for the faction Isengard:
In the movies Gandalf tells Elrond that Saruman has cross-bred orcs with goblin-men. This may have made the Uruk-hai in doing this. Goblin men may be a cross-bred of Goblins and Men, Men and Orcs, or Uruks.
In The Two Tower video game Uruk scouts are green. They could be Goblin Men. Uruk-hai in the game seems to be greenish to. Lurtz is blackish brown, so all the Uruk-hai in the game, or all the scout uruks could be goblin men. In the Amon Hen level Legolas says that they must defeat the Uruk-hai when the entire enemy's are scouts. Goblin men could be a kind of green Uruk. Also, they could be Half-orcs.
I was thinking, maybe this could be used as some kind of fast light infantry unit for Isengard?
"Wild Dwarves" for the faction Rhûn
Rhûn was the domain of the Easterlings, Men of Darkness who were ready to follow both the Dark Lords and fought as their allies in war. These lands, too, were peopled by lost Elves, Avari and Úmanyar, and by four of the seven clans of the Dwarves.
I was thinking, maybe there should be like 1 or 2 Dwarve units for Rhûn, but very expensive ones. So that they are only used very few and cannot be used too much as a "counterpart" against the Dwarve faction.