Maybe he gets reborn or something and becomes a Robin to Dick Grayson (and Damien dies?)
That's not so far-fetched. Something very
similar happens in The Resurrection of Ra's Al Ghul
-- Ra's has been reincarnated as a zombie (of sorts), and he needs to transfer his spirit into a compatible body. He wants to use Damien's body, because Damien is his grandson.
The precedent is there -- Bruce could need a vessel to store his spirit, and Damien's body would be compatible. In this case, Damien might actually be a willing host, because he venerates his father.