I like a lot of your new villain ideas, but I disagree with the bit about Surtur. Aside from being the big bad in Simonson's run, he is a woefully underdeveloped character. Surtur and the entire realm Muspellheim have barely been touched except for the odd event motivator. But instead, Marvel has chosen to just make a female Surtur-lite with this Sindr character. Why develop Surtur more when you can just toss him in the bin and make a female knock-off? Sounds like another stellar Marvel idea. Also, while he may look like it at first glance, Surtur is not a demon or a devil and Muspellheim is not some realm of damnation filled with demons. I really wish Marvel writers would understand this. Surtur is the living embodiment of cosmic heat and flame. There is more potential for the character with this understanding in my opinion.
As for Jormungand, he/it is really not even interesting as an idea to me. I haven't been a fan of any version Marvel has done of the character. It's really just supposed to sleep until the end of time.