Nebulous
Legend
Nebulous, that is really wicked. I finally sat down and read that.
How does the Frog Queen instigate Thunderspire, by chance?
Well, i'm haven't decided how much the Frog Queen will play into Thunderspire, but i think i'll use her as a recurring NPC who pops up from time to time.
Another idea is replacing one of the encounters in KotS. In the Blue Ooze room, I was thinking of instead having a door of bolted iron. The PCs hear sobbing on the other side, a little girl's voice, begging to get free. And if the PCs don't let her out once, twice, thrice, the voice BOOMS and the walls SHAKE, dust falling down. If they do open it, a great wind blows, and they hear laughter, and then nothing. There was a faerie bound in that room. What, I'm not sure.
I used Sir Keegan's murdered daughters as ghosts haunting the ruins of Shadowfell Keep. You could use a similar idea maybe and having the little girls voice to really freak the players out. Now, my ghosts were harmless, but you could easily make them formidable foes if you wanted.