My immediate seed once I heard about the plague lands was a dark circle of druids, extremists laying waste like to civilized lands... the second seed was (D&D terms here) was something akin to the Far Realms, or if you prefer, a lovecraftian realm where madness and chaos rule, has touched somewhere in the epicenter of the plaguelands... or, you could even spin those two together where the druids unleashed something they couldn't control or contain and now it runs rampant having killed and decimated their would-be controllers...
but since my area is on the edge and not the center of the labled plaguelands, I'll just try to mesh my mapping to whatever effects we finalize on it...