Íluvatar's opponents?

[Legolas]

So, Eru needed Morgoth (or one of the other Valar) to turn to evil so that his work could be appreciated. But then I guess he's probably not so vain as to feel the need to be appreciated.

No. He didn't *need* and doesn't need anything, nor will he ever. Any being doing good or evil - any being doing what he intended it to do, which is "being" - is fulfilling his plan. It only shows us that his design is perfect. He knew it from the beginning.