I think if Brock and Misty were returning to travel with Ash again that would have been revealed also and it would reek of desperation for garnering more attention to the series and attracting more adult viewers that watched when they were younger. I can't see the production team bringing back them or any other classic characters for anymore than a 1-2 maybe a couple more episode appearance in SM because it would be denying this series having it's own identity and there's already a sizeable established cast, but perhaps a reboot of the original series is in consideration after SM ends.
All we know right now is the Alola gang is travelling to Kanto and Ash will reunite with Misty and Brock for two episodes. Maybe they or Ash's later companions will be back again in the future and we'll find out later when the airdate for their next episode(s) get closer. Right now we know what we know and Brock and Misty's guest starring is getting publicized because it's a big deal and it's something that's exciting for most long-time fans.
It was also just as much of a big deal when Misty returned for the beginning of Battle Frontier, and also just as much of a big deal when Charizard returned for Best Wishes. Heck, Ash even doing the Battle Frontier in Kanto of all places, and close to the franchise's 10 year anniversary no less, would have been just as big of a deal as this. Last I checked, they didn't get much press especially compared to right now. Heck, Dawn wasn't even ANNOUNCED when she made her reappearance in Best Wishes (and believe me, if they had announced Dawn reappearing, we would have known a while back via Bulbanews). So no, I'm definitely sure their appearance is going to be big. They had plenty of opportunities to make a highlight, including, I should add, with Battle Frontier where Ash was travelling through Kanto for the titular event, yet they had a lot of missed opportunities to actually make a big deal of it, why make a big deal now?
Besides, them even changing the overall nature of the show to be a Yokai-Watch clone with SM in the first place reeked of desperation anyways, so that shouldn't be a reason to dismiss this as reeking of desperation especially when this at least actually pays respect to the original series far more than M20 did.
Now, I'm not saying that Misty and Brock are permanently returning, because we have no real knowledge yet. Even with the huge amounts of teasing, it could be temporary. The only thing I can definitely say for sure, however, is that the amount of promotion they're giving for them is definitely FAR more than just for two episodes (they didn't do this level of promotion when Misty returned for the Mirage Kingdom arc. They didn't do it for when Misty reappeared at the start of Battle Frontier, they didn't do it at this level with the Pokemon 10th Anniversary Special, they didn't do it to this level with May's return for the Wallace Cup, they certainly didn't do this at all with Dawn, heck, they didn't even do that with Charizard when he returned late in Best Wishes, and that was the time they outright averted him returning to Charcific Valley. Why give this amount of promotion if they were only going to appear for two episodes, when they didn't do that the prior times they appeared, even in Battle Frontier or the 10th Anniversary Special, which would be the closest comparisons to this?).
Oh, and BTW, some more teaser have been released via Oh Oha Aniki!: One has Misty pulling Brock by the ear, the other has Jigglypuff singing (yep, Jigglypuff's back, marks the first time it appeared since AG with that Whismur). Another image has Ash, Pikachu, and Misty reuniting with Bulbasaur (well, kind of regarding Misty, since she's partially cut out of the shot). The last one has that black guy riding a Charizard while pursuing a Rapidash. Also, there's a closeup of Bulbasaur looking giddy (presumably seeing Ash for the first time in a while) and also what's presumably a flashback to Ash meeting Larvitar, implying that we might get a chance to see him soon. And a new clip of Brock has been released that matches up with the storyboard art that was posted on Tumblr. Also, it's made clear that the scene with Misty and several hearts were from Ash's recollection of her.