My first thought on 'The Rite of Spring' was....oh no, don't use anything that was used in Fantasia! But that piece is 35 minutes long, and listening to it, somewhere around the 16:20-20 min mark sounded like a bit from the Star Wars soundtrack (on Tatooine). So....I think we are safe to use pieces of it as inspiration for an original score. I completely agree with you that it has this air of anticipation to it, and much of it is ominous.
Dance of the Knights is very jarring, so it's got that going for it. Melkor doesn't play nicely with others....
Many classical pieces have these slow quiet parts that build up to something really bombastic, so I think Melkor's 'theme' could have a variety of moods to choose from, with only some of it being the 'evil overlord cackling from on high' levels of drama.
I feel that Danny Elfman did a good job with the 'ominous background building to frightening crescendos' in his soundtrack for Red Dragon. Much of the soundtrack is quiet, but it doesn't take much for it to dominate the scene. And, yes, it's modern serial killer stuff, not mythic wars, but still....good stuff there.