Not to burst your bubble but Misty goes back to her Gym and Brock back to training to be a Doctor. He does act like his usual self around Olivia though. That being said, I don't see how anyone could expect a different outcome than Misty going back to her Gym duties.
Ah, let's see, there's the fact that in her bio for her first appearance in SM in CoroCoro, they specifically listed her as Water Type Specialist, not even MENTIONING her status as a gym leader, the fact that Brock, who, BTW, was repeatedly stressed to have been a former Gym Leader in that episode, having an overall better battling track record in that episode than she did, even WITH her nigh-insurmountable strategy that Ash overcame, the fact that she actually seemed to have enough faith in her sisters' capabilities to actually let them run the gym without worries, both of whom for the record redeemed the Gym Leaders' honor after the group as a whole got treated as jokes since AG up to BW (bear in mind that in AG, she actually had to be forced off of resuming her journey because Daisy effectively threatened to hand out badges), and heck, the fact that they were actually added to the opening recently (and not just added her in, they added her in with specific clips from those episodes), something they have NEVER done in past series since her departure (look at AG. Did they add her to the opening during the Mirage Kingdom arc? Or heck, the beginning of Battle Frontier? Absolutely not.).
And let's not forget, her goal, back before Gotta Catch Ya Later pretty much threw a wrench into everything, was to become a Water Pokémon Master, which based on what was said in past episodes entails trying to reach levels comparable to the Elite 4 at the very least. That would naturally REQUIRE traveling to accomplish, and certainly cannot allow her to stay as a Gym Leader (heck, Ash never stopped his journey to become a Gym Leader, or even a Frontier Brain). There's even an essay explaining this bit here:
https://www.pokecommunity.com/threads/336960 (heck, forget her WPM goal, even Flint, as in Brock's father, pretty much implied being stuck in the gym would cause a trainer to atrophy in skills based on what he said about Brock). She needs to resume THAT goal, which was her dream before her sisters forced her out of that goal.
Heck, in Best Wishes, we probably expected Charizard to fly right back to Carla and Liza at Charciffic Valley and resume training once his time was up, yet guess what? He stayed for the remainder of Best Wishes, and actually got upgraded to Oaked Pokémon status. If they can do that for Charizard despite patterns indicating he'd go back to Charciffic Valley once his usefulness is done, they most certainly can do that for Misty.
Well, at least we'll probably get one more appearance from those two considering their appearance in that teaser poster had them in their Kanto clothes and not their Alola clothes, and unlike Nagisa whom they edited in that poster to include its haircut, they didn't even bother altering Misty and Brock's outfits to take into account their Alola outfits. Let's hope Ash at least wins the Alola league (and there will be an Alola league anyways. SM043 foreshadowed it).
EDIT: BTW, there's a line of dialogue that indicates that this definitely won't be the last we see of them in this series. Here's the line:
In case the image doesn't display:
https://mistyisbae.tumblr.com/post/181793038922
![[PokeCommunity.com] SM 103](https://archives.bulbagarden.net/media/upload/thumb/8/88/SM103.png/267px-SM103.png "[PokeCommunity.com] SM 103")
![[PokeCommunity.com] SM 103](https://otnesse.tumblr.com/image/181793760979 "[PokeCommunity.com] SM 103")