I like the thought of the Celtic-influenced opponents as well, nice change from stock Cthuloid critters.
I have also being looking for some nice (read 'nasty') Fay folk to tinker with as well to include with this theme.
Otherworld Miniatures have a Leprechaun that would be fun to use in a game - not so dangerous as many, but jolly well annoying to the Threshold player.