As for uniqueness and unexpected, I don't see GF minding to keep something usual if it fits the Pokemon concept. Ho-ho is Fire/Flying and Palkia is Water/Dragon and, unlike the 4th and 5th gen legendaries, Yveltal and Xerneas don't look like they would bring up any new combinations to the table.
I didn't read your neuron/muscle theory btw, but from what I've read from people here, it didn't look realistic. Unless I'm missing some important arguments.
If it turns out to be Fighting/Flying, I'll give you extra 50 cents. And a ribbon. xD