I'd love to see how the
Madspark Eccentric plays out, although it might need to be an NPC ally in order to actually use its abilities without just getting rolled (it definitely won't work as a single enemy).
As far as villains go, I'd
adore having someone give me some critique on the
Siedkona of the Iron Bands, although using one of those as a one-shot baddie is, in my mind, a waste of the potential plot behind the class. If you're interested I can write up a stat-block, but the Seidkona is really only BBEG appropriate (due to the world-altering nature of the Fey vestige binds), and I don't really want to run away with your plot.
Oh...and the
Whimsycap Toadstool, although you'll need a group of them in the midst of another encounter to challenge PCs of that level.