Ooh, what about a magical plague that leads to zombies? Three ideas in one! Though how exactly that'd work would be something we'd have to figure out. I technically accidentally laid in some groundwork for a plague already, what with that sickness I alluded to with Leland.
Maybe some people didn't come for healing, and it got worse and is spreading. Or it's changed somewhat and now it's a magical plague that either drains, negate, or otherwise changes abilities?
BRAINSTORM! Anyone see Misfits? Where there was that drug that made everyone's powers the opposite? For instance anyone with reading would instead be projecting thoughts. We'd have to figure out a good opposite for some of the talents though since not all would be obvious.