In reality everyone should return.
Dawn,Misty,May or Brock played important part in shows history contributing to its popularity, they have unfinished things and getting some recognition serves as nice gesture from writers side adding that extra spice to show.
Return of previous characters also provides more references and flashbacks to past strengthening continuity of pokemon series, and they could become relevant again if writers decide to bring them back to life.
Most other anime which replace characters tend to bring older ones back eventually to develop them further and make relevant to story again helping to advance it with new/fresh things getting actually applied, so i dont see why pokemon wouldnt do the same?
Speaking of like hood of this happening, naturally Dawn and Brock would probably have priority given how they were recently replaced characters.
I could see Dawn popping at some point to either help promote remakes or come in filler arc to help promote idea of contests among people,seeing how they dont seem to be there.
While with Brock it wouldnt be unusual if he traveled to gain new knowledge and learn from other pokemon specialists, which could help him in his future carer of becoming doctor.
I could see him and Ash meting on road at some point.
About Misty unlike some people i dont think we will never see her again,and imo its little naive to think how anime will end without character coming in person ever again.
She is one of most popular characters out there,there are many things left open(lik her goal of water master,ambitions,backstory etc) giving writers material to use her and her unexpected return could serve as nice twist bringing more continuity within show.
I cant see why would writers miss on opportunity to receive boost in TV ratings.
Judging by past character cameos,they usually attract more of viewers toward anime.
Misty and May appearances were one of highest rated episodes for AG and DP for a reason.
I could see her coming back to either compete in some tournament(mot suitable would be water based),giving us opportunity to see how much she improved skills as trainer,to show her new pokemon and advance goal of water pokemon master.
Playing some role in Iris growth as character isnt something farfetched to expect either,given how this two characters have quite a few things in common sharing some characteristics with both being connected with gyms.
For instance around Opelud city, having Misty helping Iris about possible issues regarding Drayden and 8th gym would make sense. Similar how May helped Dawn about confidence issues in Wallace Cup.
As we all know Unova is portrayed as far and isolated region with older pokemon not migrating there due to distance. For same reason its very hard to get 5th generation pokemon out of Unova,so having Misty travel there to learn more about new water types and catch some of them makes perfect sense.
Imo Misty will always have chance to return because she was part of first cast which made anime popular and recognized in first place.
Ash,Misty and Brock are šprobabl most iconic pokemon characters,demand for reunion is huge,and judging by other TV shows first group of characters sooner or later is bound to return and get some kind of recognition.
It doesnt matter if she didnt appeared in DP,that doesnt guarantee she wont in some future region with writers already proving with Gary or Jessibelle return after long absence,how its never late to reintroduce someone.
As for May assuming remakes will happen around time filler arc comes,gives her chance to be used a on of candidates to comeback helping to promote them. There really wouldnt exist purpose in introducing someone else to cover up potential remakes when May is already more than suitable enough for this job being iconic to that generation.
Unfortunately remakes doesnt always guarantee that older character will return. Writers could have used Misty for HGSS remakes for example but they passed on that opportunity.So partially May or Max are in danger of not appearing either if Ruby and Sapphire gets released.
i would like for misty to come back, just to annoy so much people, but may will be the one returning.
Seeing how large majority wants or wouldnt mind Misty coming back, the only fans which would in reality be annoyed are small vocal group of haters.
Its generally accepted how Misty left before her time was up, having lot of untapped potential left to be done more with character. Demand going for character return only proves that.
