This in all honesty would most likely end up being a really cool thing to add into Minecraft. I feel that the Phantoms should have their own realm. They just seem like they would be that kind of mob to have one in the game. Maybe call it Limbo or something like that. Within this realm of the Phantoms should lay a boss. After defeating said boss, a castle kind of structure should appear close by with a couple chests filled with some good loot. Kind of like how there are the End ships. In order to get to this realm, maybe make it so that those phantom membranes are used to create a type of special block meant specifically for entering Limbo or whatever it should be called. The shape could be that of a circle or a diagonal square (aka diamond).