The Treason of Isengard: Eru's Plan?

[burrahobbit]

Could it be that this new theme "inserts" the changes in the Music of the Ainur he intended for Men to do?

Possibly. I doubt it gets into specifics though, as that would go counter to the whole freedom thing that Tolkien talked about. It might be "Men will be important for this andthis reason" but it is impossible to say really.