Abyss only needs a Boss, other than that it's finished.
I highly doubt Phantom School will ever actually be used for anything as it's -completely- neglected in Trickster and always has been. Heck, the place doesn't even have Shadow monsters or anything. Mad Ray is basically considered a normal monster even, not a boss.
Phantom Dungeon was just something thrown in there to give it -something-, but meh.