Why are you talking about "people" as if its everyone? Plenty of people never had any interest in seeing Misty return and felt she was a done character.
Not everyone but huge portion of fanbase wants this to happen with demand still going big with number being counted in thousands judging by what you can find on youtube,various pokemon sits,petitions etc,etc.
Also for record it was already established among older fandom whether they want or dont want that she returns how Misty as character wasnt fleshed out enough never receiving closure to story having many things unanswered and unexplored like goal to become water pokemon providing lot of unused material to follow up on it).,having untapped potential to do more with her pokemon team,to expand more on back story(like what happened with parents,from where fear from bugs originates etc),developing her more as trainer.
There exist noticeable differences in Brock's current goal and his previous goal. I don't think pokemon centres would hold Brock back from travelling. If anything he can travel to other pokemon centres to learn new things, handle challenging cases, training purposes etc and even meet Ash and company at some random pokemon centre. Plus we don't know why exactly Brock left Professor Ivy. All I remember is Brock geeting curled up in depression everytime someone mentioned Ivy's name.
At this point it depends on Brock whether he plans to open pokemon center or become traveling doctor but it cant be denied how some differences between breeder and doctor carer exist with both involving healing and taking care of pokemon keeping them in top form.
About OI issue it was implied in episode "Tent Situation"if im not mistaken how reason why Brock left prof.Ivy is because she dumped him.
Its of course only assumption being based on what it was said in that episode by other characters.
I'm not against the travelling of Brock. He definately deserves to travel places, explore regions, meet the best doctors of the world etc but it's him travelling with Ash which I'm partially against at. He has already travelled 4 regions on-screen which IMO is more than enough. None of the main girls/rivals got that luxury. I just don't think both of his goals suit him to travel with Ash and gang. IMO the writers can't do much with Brock's goal and they have demonstrated that by neglecting Brock in Johto, Hoenn and Sinnoh(compared to female companions of Ash in those regions).
Reason why writers didnt do more with Brock is because his goal of being breeder was rather passive not being connected with battling or entering competitions as well there not existing any benchmarks through which it could be showed visible progress kinda stopping whole thing in track.Competitions arent necessarily though they help,but when your not a battler either this really creates a problem to have someone become more actively involved.
Even before female co-star got introduced to cast with Misty being around,Brock always received the least amount of screen time and passiveness of his goal was probably main reason behind that and why nothing changed.
For what Brock goal was being mostly about taking care of pokemon,raising them to be in top shape,preparing food etc he got some pretty good screen time displaying his knowledge and skills as breeder quite a few times gaining some development,raising pokemon egg and taking care of couple of baby pokemon becoming recognized by others fir his knowledge in that area.
However there is only so much you can do with goal which doesnt involve competing or battling.
His new carer of becoming doctor at very least provides many alternative ways through which it can be advanced doing something worthwhile out of it so some good development could be done if he joined Ash again.
As traveling doctor he would learn and improve just as much in company of his friends like he would if he traveled alone having in either way opportunity to explore new regions,pokemon and gain experience out of it.
I think Brock has matured as much as he could have in the show. The only thing which remains is showing Brock being able to control his craziness on seeing pretty girls. Brock's character failed to contribute to group dynamics in sinnoh (IMO at least). Presence of Brock doesn't guarantee existence of better group dynamic and chemistry among group members IMO. It's not that I'm completely against the return of Brock to main cast. I just think that if anything characters like May, Dawn, Misty and Gary deserve a place in main group more than Brock. I wouldn't like either Cilan or Iris to be replaced any time soon at the cost of Brock either. Because those new characters deserve some love too. However, I'd like Brock to return as a travelling companion over the introduction of another brand new travelling companion.
I still think writers could revamp Brocks character to make him more interesting by revealing more of his past,starting to focus on some of his interest,introduce missing links from past and issues which werent brought up before.Brock had potential as character and still has being possible to flesh him out more but unfortunately writers left it go to waste instead of utilizing his character properly.
Talking about Iris and Cilan they will probably last for four years and be replaced like previous characters dud unfortunately,After that imo writers should stop with introduction of new characters reusing some of older ones developing them more instead of laving them rot in past with unresolved stories and being only halfway developed.
In episode called "School of hard knocks" it was shown that there exists an alternate path to enter in pokemon leagues without travelling places and collecting badges. The problem with Misty's goal is that it's probably the most vaguely defined goal anime-wise(out of all the goals of main characters of the show). We don't know what exactly a trainer has to accomplish in order to become an E4 member specialising in a particular type.
It would be unfair to absolutely rule out existence of alternate ways(which we might not know as of now but could be revealed later in the show) to become a water pokemon master. Possibilities are many. Out of which I think it's possible that once Misty becomes strong and enhances her reputation to the best gym leader of kanto through some sort of rating system run by Pokemon league committee. She could get a call from Pokemon league to participate in a series of water related events on winning which(or coming close in doing so) she can pursue her goal of becoming an E4 member or water pokemon master. This of course is merely product of my imagination. Since Misty's goal is similar to Ash I do think that alternate ways exist for Misty to become water pokemon master which might involve less travelling. Through what path Misty wants to become a water pokemon master is another thing.
Koga, who was shown to be an average gym leader of Kanto in anime was later revealed to have become an E4 member of Johto in Gold/silver/crystal. I don't think that how he achieved this was clearly stated in anime though.
Well, Misty is gaining more experience as a battler by battling trainers who come to challenge Cerulean gym anyway. So yes I do think possibility might be there that Misty is getting closer to her goal by staying at Gym at slower rate however.
Misty goal isnt so vague when you think about it,it involves becoming best or on of best water trainers in world which are basically E4 members with some being labeled as on type masters.
Ash goal is probably even more vague since its unknown what else is required to accomplish except winning league and beating E4 and champion considering how even Cynthia and Lance arent pokemon masters.
Becoming master of on type like E4 position showed is easier to accomplish than Ash quest of becoming best trainer in whole world mastering all types.
Way in which it can be achieved may seem vague but whole thing got defined little more during Whirl islands introducing way through which someone can advance this quest.It was stated by prof.Elm during Whirl Cup in episode "Around the Whirlpool"with more details being given by water priestess Maya,how winning this tournament gives you title "Alpha Omega water types" by which water masters used to be called in past bringing you closer toward dream of becoming water master.With Whirl Cup writers showed how participating in water based events winning them,it needed in order to achieve Mistys goal.Events like Whirl Cup from Johto basically showed how there exist some special tasks which someone needs to enter to become master of water types(similar how coordinators needs to do contests or Ash gyms) .
Reason why follow up didnt occurred braking new grounds with it was because Misty already left cast.
Since other than Ash case of becoming pokemon master there have been no introduced alternative ways in becoming top pokemon coordinator, pokemon connoisseur or now in Iris case dragon master judging by what it was showed with Misty quest of becoming water master there most likely doesnt exist alternative way either.
Otherwise we would heard about some "other ways already"for other careers.
Problem with gym is that is Misty cant become water pokemon master in this way.I agree how at gym she can learn some new things and techniques but being gym leader can only get you so far,with trainer after that starting to stagnate not being able to learn more with his potential being hold back. Misty wanted to become best water trainer and strongest water trainers are probably E4 like Lorelai not being possible to become that strong at gym.
Many of them as we get to see are at said position for their whole life basically(like Blaine,Wattson etc), after all this years of battling and training still not being even close to Frontier Brain or better yet elite trainers level.
Reason is coming from lack of experience.
Gym can only get you so far with key for further improvement lying in leaving it to learn new things and improve as trainer outside of it.
Thing is that by traveling Misty could expand horizons being possible to meet stronger trainers,other water trainers which dont come to gym,learn new things about water pokemon much more by observing them in natural habitat than she could ever at gym as well entering tournaments to help her to improve skill.
Many gym leaders which became something more left to travel to learn new things and improve because of gym not providing them with enough experience and knowledge.Like Wallace for example who left gym to travel and improve before becoming champion,contest master and one of best water trainers in world(see connection with Misty?).
Also Whirl Cup proved in a way how there exist prestige competitions through which her goal can be advanced which wont com to her if she stays at gym requiring to travel to enter them and gain new knowledge.
Writers introduced already way through which goal of water master can b advanced,only problem is thy never went beyond that.
From what it was showed in original series Misty goal was showed to be similar to Ash requiring traveling,meet strong trainers and learn about pokemon in wild along with advancing it through tournaments/competitions.
This was main reason why she left gym to travel in first place according to 7th episode with purpose being to advance her goal for which gym is nowhere near enough.Only reason she went back to it was because she was left without a choice.
Because of above mentioned reasons and inability to become that strong rceiving said title gym doesnt have any connections to water master thing and in best case can only serve as "stepping stone"not being clearly enough.
Speaking of Koga in anime we dont even know if he became E4,however since time skip of three years passing never happened in this show like it was case with games Koga is probably still gym leader.
Also it was referenced in games how there years passed between leaving his position and becoming E4 probably going on training in meantime.