It would be pretty dark for Julie and the Phantoms to reveal that Nick was killed when Caleb took possession of his body, although I do think it would be funny to see Ghost Nick try to join the 90s-tastic Phantoms.
Another fun possibility would be for the possession to leave Nick with the ability to see ghosts, including the Phantoms.
They wouldn’t go to Caleb’s club to save themselves, but they might do it to save Julie.
Nick is a good way to get at Julie even if she prefers Luke’s dreamy dead eyes, and Julie is a great way to get at the guys.