Interesting! I can't help but wonder if maybe that's Decade!Mio (or Yuki, IIRC). Also, it amused me that it took a good 5 minutes before I realized that this was a continuation straight off the original show, since you've got so many AU's and I assumed immediately this would be related to one of those.

(of course, if she comes from another world, there's no reason it can't be related to one of your AU's eventually)