Well, I can’t really speak for what they will do, mostly since I don’t think they’ll remake the second generation games again. I can talk about what I think they should do, or what I would do – which is to say “f*ck it” because what Whitney likes are “cute Pokémon” and Normal and Fairy both have those in abundance. It’s not like she’d be the first major opponent to look outside her speciality; Sabrina has a Venomoth, Candice has a Medicham, Volkner has an Ambipom and an Octillery, the Striaton triplets all have Lillipup, and several Elite Four members are all over the place. They could even reference the fact that Clefairy was once thought to be a Normal-type until Fairy Pokémon became better-understood.
Also, there’s a fourth possibility you haven’t mentioned – make Whitney a Fairy-type specialist, keep Miltank as a lone Normal-type and give her a different signature Pokémon, like Clefable. Why not?
