But Middle-Earth isn't balanced. Gondor has nothing to counter the Nazgul with, and Rohan has no Saruman. That was one of the themes of the books, that the Shadow was much stronger than the free people, yet the free peoples still won. To make the free civs equal to the shadow would get rid of the desperation the main characters in LOTR felt. If Tolkien's Middle-Earth were balanced, there would have been no need to destroy the Ring, because they could have just attacked Mordor.
So, my idea is, to make it possible for the 'free peoples' in the mod to win against the shadow, they should have more religious bonuses if they pick a 'good' religion, and maybe, once in a while get powerfull units, such as wizards, or eagles. They basic units should be more powerfull than 'shadow' units, but more expensive, and basic 'shadow' units should be weaker, but cheaper. So there could be large 'shadow' hordes who could overpower the 'good' units, but the 'goo'd civs could have very powerfull units to counter these.