It's not a spin-off starring Ava / Rebecca / Jane Doe, but Elizabeth Reaser, one of Grey's Anatomy's many memorable guest stars, will play the lead role!
The series, if it gets the green light from execs, will zoom in on Elizabeth Reaser's character (named Annabel) going backwards through dating timeline.
Mythological X will air on Fox should the pilot get picked up. Right now, it's only in the "sell, sell, sell" stage of TV production. Hopefuly it works out!