there was lore that a ghost thought he was about to revive a fallen. So the ghosts themselves lorewise do not know if that is an option. I think there is also lore about how ghosts could revive people to make a guardian who where not their perfect match. I figure it is possible. I have made up a story for a comic i was making durning destiny 1 about how a fallen had saved a ghost from the hive devouring it's light, and had chosen to revive the eliksni for her sacrifice in saving the ghost. She had to develop her own powers and visited places our guardians had traveled and done their deeds, looking to learn what it ment to be a guardian.