First of all, are we absolutely certain that Disney isn't significantly expanding/reworking Maelstrom to make the Frozen version bigger? The rumor I've heard is that the current unload area will become both load/unload and the current load area and last 3 switchbacks in the queue will be a new show scene area. Assuming that's all, it's not going to be enough, but are we certain that's all?
Second, while Fantasyland is the most (if not the only) appropriate location thematically, the Magic Kingdom doesn't need any new rides right now, certainly that would boost attendance even further. If anything, that park needs some low-key, but high-capacity additions to relieve overcrowding.
It's the other 3 parks that need the Frozen bump, but MGM already has the "Summer Fun" and Wandering Oaken shop or whatever, and supposedly there are major expansion plans in the works involving Star Wars and Pixar, which are probably 8-9 years out, but still coming. Animal Kingdom would be an even worse thematic fit than the World Showcase, and that's saying something.