Hmm. Time themes idea seems to be fun, but it really restricts imagination.
Especially that magic era=>modern age=>units era idea.
'There was more magic in the past' tune is classical, but 'magic continues to wane' is something I never really liked. I dont think it's a must.
For example, Man - right now you can assume both that the Sidhe are gone for good and that they have returned. If I got it right, they are going to become 'the Past' now.