It may be possible
well am not sure weather you can do this but
the elves could have oil spillers that put down a sort of flamable oil in a area and when a fire arrow attacks the spill it ignites and kills troops instantly in that area.
A new unit for the elves could be an elven shieldmen. they are slow, think of the gonder tower guards but faster, but have high defense and a moderate attack. their main purpose is to combine with archers and provide an exellent defense. they are not meant to attack head on but more of stay back and defend the base kind of unit.
Love the idea, and can see a way to make it work
For the goblins, shelob could have a spell that she can lay spider eggs. once they hatch, they burrow in the ground and all you can see is a web. if enemy units walk past them, the spider will come up and eat them then die. its like a one time deal thing.
Isengard has enough units, I think
for isengard, you could have an uruk scout unit that is fast, has a large vision range, semi-moderate attack but very weak armor and can blend in with the trees. he job is to spy on enemy troops.
Mordor has enough units, I think
A unit for mordor could be the fat orc with a giant mace like in the return of the king videogame. he could have an ability that will damage surrounding orcs but increase their speed, attack speed, attack, and defense. He could also have high defense and attack, but be slow.
I'll do it, but for Hama
You could have theodred as a hero for rohan and could be llike the royal guard captain. For level 1, he could have a leadership bonus and mount. for level 3, he could have blademaster. For Level 6, he could have an ability called power slash. its just a regular slash but more powerful. Finally at level 8, he could summon 4 battalions of royal guards.
I'll think about it
For Gondor, you could have a spell that could Increase health and defense of all buildings, make the trebuchet more resistant to attacks and have more health, and make the walls made out of numenorion stone. it could be called wonders of numenor. someone else could rename it if they have a better name.