I wouldn't be against it. We haven't seen Brock and especially Misty(entire decade passed) for a very long time. They are important piece of pokemon history, most iconic characters after Ash being there when everything was just starting out helping in becoming pokemon anime worldwide accepted and popular in first place.
So do Ash very first traveling companions and friends deserve recognition? Follow up? Most definitely. And with pokemon anime reaching milestone of 20 years of its existence, aka 20th anniversary i wouldn't rule out possibility of both or one of those characters based on adaptability with current region coming back(its not hard to add together 2 and 2 and see why Misty/Kasumi would benefit ALOT from Alola islands giving foundation so that her comeback could be easily constructed in unforced , natural way).
With so little continuity present in pokemon it would be really refreshing and entertaining to see more ties to past and return of familiar faces taking them in new, exciting directions. Serving as bridge to link Ash to his previous regions and achievements, explore on forming dynamic with new characters and renewing friendship with Ash deepening on it. Making new kids familiarized with Ash past to better understand this anime wouldn't hurt either.
Not to mention im pretty sure most fans would like to see how much Brock and Misty have changed in personality and even possibly looks. What new pokemon they have, how much they improved, what new adventures they had and how are their goals going. Receiving continuation of their by this point forgotten stories.
Both these characters are done. Maybe as cameos but not as main characters.
This was evident when Misty got stale in Johto and Brock was stale in Hoenn, relegated further to plot device keeping Ash and Dawn alive in Sinnoh.
I think way in how was Misty abruptly removed from main cast followed by unclear situation in chronicles taking over gym due to sisters laziness(not having other choice at that time), with character showing desire to hit road again pretty much demonstrated how Misty story was indeed left unfinished having room to do much more than what was showed.
From her dream of becoming water pokemon master, unanswered things about past , her parents, to learn how to use pokemon to full potential(such as Gyarados or Psyduck). Or lets not forget desire to reach E4 level of power being highly interested in them(idolizing some like Lorelei) wanting to make name fir herself as water specialist on global level opens whole new area worth exploring.
And such a water rich region like Alola is idea, in fact perfect place to bring back water enthusiast and aspiring water specialist like Misty back making something worth watching through sequel of her story.
In Brock case new career of becoming doctor just by itself soils ground to make whole new story centered around him, but this time better written. Because character is released from constraints from past being given with new goal, new clean slate to make something good out of it. Since there do exist traveling doctors who travel to various places to offer help, meet other specialists, learn about new potions, methods of healing, pokemon etc.
Now whether they became stale or not is purely subjective(this could be said for literally every pokemon character if viewer didn't received enjoyment from watching something or someone). Maybe Brock did hit ground in Sinnoh, but in Hoenn? Nah, far from it. Especially when Brock received a lot of action there, entered several competitions, established highly enjoyable dynamic with May and Max and his breeding skills were displayed on numerous occasions. More than i can say ever happened in Johto. And specials after his removal with Cilan or when helping his brother Forest to become gym leader made it pretty clear how this character still has juice left in himself worth following if given proper attention.
Same applies to Misty. Because even if she may have seemed stale at some pint in Johto, that definitely doesn't apply to last Johto season master Quest, followed by chronicles and dealing with Cerulean gym. Or cameo appearances in Hoenn when releasing Togepi and helping May to reconcile with May. Because Misty story moved forward, she had direction through which character could be taken, there were numerous highly enjoyable relationships formed with other humans and pokemon. With character being highly lively and full of spirit with newly born maturity coming from character development adding much needed extra layer of depth to her characterization.
Something i liked about her adding new interesting traits to her personality(like being more playful, sassy in different more lighthearted way or abit arrogant when you push her buttons being provoked by others with gym responsibilities and vain sisters clearly grinding her gears at times, while keeping those older appealing ones as well.
Likewise just like something enjoyable can turn in boring. Stale can also become interesting again going in circle. There is no one way direction in literature.