You've created Series X. It's a fanfic of Series Y. You've created an Ascended Fanfic.
An Ascended Fanfic is a work that's technically a fanfic, but it's made by a genuine writer, thus making the technical fanfic an official piece of fiction. Two possible justifications:
- Series Y is in the public domain, so it's legally possible to make a blatant rip-off.
- You asked nicely for permission, if Series Y isn't in the public domain.
Remakes and retellings do not count