Well, it's true that Tolkien foresaw that evil would arise again, but it's not true that he believed in a yin-yang balance between good and evil, in which each depended on the other for survival. It's clear from the writings of the Dagor Dagorath that evil (personified by Morgoth) would have an ultimate ending.

Chould there be other evils from far out in the east, that Tolkien does not go over or is it just something that we must fill in with the greater part of our imagination.