Before we digress to the side-series possibility, I'd like to discuss my idea some more before the cat is out of the bag. I think that the opposite-gender character in B2W2 will be the protagonist's doppelganger in the other universe. Hilbert and Hilda are special because they don't look the same in both universes, just like Kyurem.
This could be applied to a new set of characters, but it would work so well with Hilbert and Hilda because it would explain why the opposite-gender character wasn't shown in the previous games (outside the Battle Subway).
This could be applied to a new set of characters, but it would work so well with Hilbert and Hilda because it would explain why the opposite-gender character wasn't shown in the previous games (outside the Battle Subway).