I disagree that Dragon will be next. I do agree with the first part of your post, about the "previously physical half of the type split". The thing that had me intrigued was all of the, and bare in mind, it could just be that we as fans are picking up on things not meant to be taken literally, but the Eevee in a plane type stuff that was posted and people grabbed on to. I was intrigued, because if Eevee does get a Flying type evolution, then the whole "Special types" argument can finally be put to bed. To me, Flying, Ground, and Poison are the most logical types to be covered. Poison jumps out because we now know that in the lead up to Gen 2, there were once plans for a Poison type Eeveelution, but instead we ended up with Umbreon. Ground has always stood out because it is, in my opinion, such an easy type to transition to, it's such a common element, and it would almost be like the total opposite of Vaporeon. Flying more so recently because of everyone else jumping on it, but also, ever since Sylveon was introduced, there have been ideas in my head of how a Flying type Eeveelution design could be done and not look ridiculous. Prior to Sylveon, I always lumped Flying, in with Bug, Dragon, and Fighting, I thought those 4 types were such drastic changes, but when Sylveon's design was shown, all of a sudden those 4 types started to finally seem less odd to me. I still think Flying is the easiest to picture personally, and I think Bug is the one that to me runs the risk of possibly stretching the line a bit.
I don't know, but I think Flying is the front runner on a lot of peoples' minds, and following after that, I feel Poison and Ground are high on the list of possibilities.