I think that's about what to expect from them. Extensions of Sephiroth's will, whether he's controlling them directly or just disturbing spirits in the Lifestream.
I think it'd be a good way to elaborate and justify things like the train graveyard ghosts, but they can also be built up more as a mystery, too. Like, what would Avalanche perceive "ghosts" to be? And Aerith? Is the Planet fighting back, is the Lifestream being polluted, are they tormented by regret?
They could be a recurring element assosciated with Sephiroth, part of the Reunion, or they could just disappear after he takes control of Jenova's body.
The only wrinkle in that theory for me is the term "Keepers of Fate" which implies a greater importance and perhaps even autonomy.