Hello Stazi,
wouldn it be more plausible with the Middle Earth universe to have say corrupted Rohirim for Isengard once Saruman conqueres Rohan etc? I mean for the evil side to corrupt the good side factions once they are conquered. The good side, may perhaps call upon ancient, pre-existing powers once they free a place from the evil power's spell by conquering it - say Saruman returns to the good side, for example, or his power is used to double that of Gandalf.

AFAIK, in Tolkien's world, evil has no substance of itself and appears to be as a coruption of good - hence Orcs are Elves fallen to the shadow etc.