Kate Walsh may have left Grey’s as a regular back in Season 3 to star in Private Practice (albeit with some guest appearances that followed her departure as regular), but at least Addison was still alive at the end of Private Practice.
Addison’s return could be as simple as Grey Sloan needing her expertise as a doctor.
News of Walsh’s return comes just a few weeks after she and Ellen Pompeo had a fun Grey’s Anatomy exchange on a viral video, which in hindsight maybe should have been a clue that they’re working together again.