Hoping Cynthia, as she is my favorite Champion. We'll probably get a quick visit from N. He has a huge fan following now and, last time he checked in, he was contemplating meeting up with Hilbert/Hilda, who is also abroad.
Now that we've got the likely ones out of the way, I wouldn't mind seeing what Cheren or Bianca are up to nowadays. Prof. Birch hasn't been seen for a while either. I doubt we'll actually see previous protagonists. They're supposed to be empty vessels we place ourselves into, so seeing one moving around, talking, and acting autonomously would be super weird.
The last time Game Freak did that was with Red in Gen2, the remakes, and in the PWT, and I don't even know if that qualifies. I mean, he didn't talk or move in GSC and the remakes, and he just followed what everyone else did in the PWT, so that's not really autonomous. They'd certainly be breaking the mold if they gave a previous protagonist an actual personality and individual motives though.