May be a long shot but I could see May returning from the Hoenn connection to the new region, but I'd also like to see Serena continue to travel with Ash as the electric siblings most likely wont wit the Gym and all. She could easily fit in as the new female trainer, ans as she's already shortened her hair it's just one styleing away and a new outfit; and she'd pretty much be the new trainer. :) I know they probably like to refresh the characters for each generation for a new batch of kids watching for the first time, but I'm sure they would recconize Serena even if by then only a few XYZ reruns air? And it would not be the first time they skipped a new girl, Kris came late but still never replaced Misty, and Hilda never even got a cameo. (Though I know they were going for a mirror to the Kanto anime with the Leaders and all but, still...)
I doubt Serena's going to appear in the Alola region. Ignoring for a moment that we don't even know if there's going to be an Alola anime, let alone whether Ash is going to be in it, if they didn't avert their tendency to swap out the girls when Iris was removed from the main cast despite Best Wishes being heavily advertised as being a fresh start, they're definitely not going to keep Serena or the other cast for the next region. At best, they'll keep Ash, and that's assuming they don't simply retire him.
And as far as May, I'd rather not, and besides, they didn't even bring Misty back for Best Wishes despite the fact that Black 2 and White 2 featured the Pokémon World Conference, and thus a perfect opportunity to bring her back for at least several episodes if not for the remainder of Best Wishes. What makes you think they'll bring May in, even IF there's a connection to Hoenn (which last I checked, there isn't. Kalos had a connection, but that's solely due to Omega Ruby and Alpha Sapphire)? If there IS an old character who definitely should be brought back, though, it's definitely Misty. Let's face it, other than Max (who doesn't even have a chance to pursue his goal at all thanks to the fact that he's A. younger than 10, and B., not likely to even fight Ash thanks to the writers' weird insistence on keeping Ash 10), Misty's the only one of the departed cast members who has absolutely NO resolution to her goal nor even a means to continue pursuing her goal, nor does she even willingly change her goal. May, Dawn, Dent, and Iris at least are continuing their journey. Brock managed to change his goal of his own free will. Even Tracey at least succeeded in his goal to meet Professor Oak and work for him. Besides, Alola's a primarily water-bound region (last region to have that was Hoenn, which was the region right after her departure), arguably even MORE water bound than Hoenn, meaning there's very likely to be a LOT of Water Pokémon even by Water Pokémon standards, so it's right up her alley, and bringing a character who isn't named Brock back onto the cast would definitely be a ratings boost. Heck, it would serve as a perfect opportunity to bring up her Water Pokémon Master goal and give a lot more focus on it than before. There's more reason to bring her back than the other characters.
As for the "second member" slot, a new Sun/Moon Gym leader of a younger age most likely will travel with Ash's group, whatever that may be. :P
I really would love to see characters return, especially side characters, but I doubt it wil happen as they kinda broke that tradition with XYZ so far.
Yeah. Though then again, we're not even sure if Ash is going to stay on the main cast. Considering how the XY ratings, merchandising sales, and box office receipts so far have been total crap, I wouldn't even be surprised if they just retired Ash and maybe even cancel the anime altogether.
