Experimenting as a ghost in PTR trying to find bugs and whatnot, I found myself following around players in the world and realizing: “I wish this person knew I was stalking them as a ghost”. I would be a lot of fun if, after being in the vicinity of a ghost for some time (say, 5 seconds) you received a cosmetic debuff with flavor text like “You feel a chilling presence around you”. Or a possession ability that works like Priest’s Mind Vision, allowing you to enter any npc or player, giving them the aura of “currently possessed by so-and-so”.
Doesn’t have to be those ideas exactly, but having some flavorful, non-interruptive interaction available as a ghost would make keeping around your dead characters more entertaining instead of just deleting them.