They've filmed twice right near my house. The Halloween episode was filmed in the St. Johns neighborhood, right by a family friends house (hers was the only one they didn't use!!)
I ADORE the beaver people (mostly because the inside of their house was done up in OSU/BEAVERS gear - - awesomeness).
Haven't they done a few episodes not based on Grimms tales? (I'm thinking the Bees, and Coins episodes) if so, why not a mermaids or other water-based creature? I suspect they may have one in the future based on where they've been filming, but I could easily be wrong.