The new film would need to explain why a Force Ghost of Obi-Wan would be appearing as the younger Ewan McGregor and not the older Alec Guinness, which had been what the character looked like when he actually died.
While the idea of Ewan McGregor appearing in Star Wars Episode IX isn’t entirely crazy, we did get a Yoda cameo in Star Wars: The Last Jedi, and some rumors have indicated the little green may be back in the next episode too, so this certainly qualifies as possible.
We did get McGregor’s voice in Star Wars: The Force Awakens though that was in combination with dialogue from Alec Gunness.
Perhaps McGregor is actually only going in to record audio again?