I can see cave of emptiness for Hoopa, The power plants HAVE to play some role, GF wouldn't add them if they weren't at least somewhat necessary. I always found it weird that Diancie looks like a Carbink, but that its crystals ALSO seem to match the Anistar Sundial...so maybe you do an event where you can perform a special evolution with Carbink and the Sundial? Perhaps the power plants also hold "locked up" mega stones that are only unlockable through some sort of event key. That could explain Mega Latios/Latias. The Ghost Girl in Lumiose may also have something to do with Hoopa.