Even though it is already so crowded, I think the best place for a Frozen attraction is Magic Kingdom. Get rid of Autotopia and add a matterhorn-type ride (although smaller), inevitable meet and greet, and some shopping (with snow on the roof). This way every time it snows on Main Street they can say something like "Queen Elsa is up to her old tricks again". And if you have to keep Autotopia move it to the Studios but make it smaller and themed to cars and put it in Pixar place. Make it more interesting with hills and fun turns and character-themed cars.
I do not think Maelstrom can handle the crowds if they made it themed to frozen. I'd also make the B&B show themed to Frozen. That way they get people in EPCOT for the Meet and Greet, Studios for the show, and MK once its built.
My .02 cents.