Long post ahead, but i felt need to elaborate on my reasoning and why Brock return wouldn't necessarily be bad idea and pointless thing to do.
Here are my two cents:
As humans we tend to be highly judgmental and ready to remember mostly negative sides of some character. But for whatever misplays there may have been done with Brock character it was solely writers fault, and just as writers can deliver to us(viewers) poor handling of some character. They can also learn from past mistakes, use older character background, established personality and foundation on which story was built and developed as guidelines. Helping directions which would allow them to look back on what worked and didn't with character they wrote for. Making their job all that much easier to avoid repeating same mistake again utilizing older character in case of sequel being made to his story better.
As evidenced with Clemont treatment in Kalos series so far, writers and people in charge can and could transfer their improvement and fresh ideas they are doing with new cast to older character like Brock as well. If they are motivated, invoke in themselves enthusiasm to write for Brock character again and find his presence could help move pokemon story forward through new role.
Granted there are other older characters i would rather have back than Brock; like Cilan,May or Misty for example finding myself more interested in their enjoyable characterization, update to see how much they changed and see them play notable role again having unfinished stories and lot of untapped potential for plausible continuation(like top coordinator dream, connoisseur career and undefined feud with Burgundy, Misty water master dream reaching up E4 level and overcoming unsolved fears and issues character has with pokemon or family).
But personally if there exists good enough, plausible reason to warrant Brock playing valuable role in pokemon anime again and if writers find way through which his new career of doctor can get good focus and growth, while contributing to Ash, other characters and main series storyline.
Than why not??
Taking a more closer look at Brock current position in pokemon universe and new, fresh direction in which writers took his career doesn't really prevent him from returning. His new dream involves to become doctor, learn how to properly heal pokemon, what kind of medicines to apply and study their anatomy. As such in case of joining Ash on journey in future aside from progressing his dream through field work and research on pokemon outside of center along with learning about new ways of curing from first hand by meeting other specialists not only relying on books, Brock could also be of great help in taking care of Ash pokemon whether they are sick, poisoned, injured etc. playing valuable role in preserving their health and top shape to live up to high challenges and battles they go through in Ash trainer growth. While navigating his way to new gyms, help him in training and about strategies.
Its doable.
And personally i actually miss Brock sometime. He was great character , humorous and had many cool qualities deserving more than what he got in end.
I still have soft spot for his character in my heart having substance, interesting pokemon, fun personality allowing to relate ourselves as viewers with his character and anything but easy past having to take role of father for his numerous family lacking parental love and compassion for great deal of his life, having to mature fast.
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Nevertheless ignoring Sinnoh, Brock had pretty good treatment as supporting character. In Original series he was of big help to Ash growth as trainer often alongside Misty teaching him about basics. How to not mindlessly rush into danger, prepared strategy for his gym battles or gave him needed support and encouragement.
While bringing lot of humor, hilarious escapades with his eccentric personality and more wise guardian like approach often being voice of reason, someone who knew how to bring stability and order in main cast when Ash and Misty fermenting attitudes got out of control during Kanto and Johto. He played memorable role in various arcs or events like PK1 tournament, entering breeding contest with Suzie, learning how through Golbat ultrasound detect missing objects and other pokemon. Or show Ash and everyone else dependance on his wisdom, cooking skills and knowledge in preparing medicines when pokemon center was nowhere near. As well showing persistance in approaching toward girls being rejected almost every time(poor guy), along with trying to learn as much as he can about breeding and how to improve shape of his pokemon.
In Hoenn generation Brock breeder skills arguably got more expanded than it was case in OS which was nice change of pace for role character played developing more in his own profession and as character. This was especially noticeable by making amends with his crazy family coming to better accept their scatterminded behavior and brimmed with lot of joy and pride in his younger brother Forest managing to become good gym leader helping him to find courage and succeed on his own.
He entered quite a few battle competitions, served as sparring partner to Ash when training battling his pokemon like Torkoal and Growyle with his Mudkip and Lotad,. Had many touching moments in developing as breeder in helping his pokemon over love issues(like when they had crush on Mawyle or in taking care of sick, misunderstood wild pokemon such as Solrock who caused drought in village). Feeling like he was progressing forward.
Only in DP i would say he was shamefully neglected, but he had his moments there as well. Contrary to some belief Brock still bursted with lot of personality in Sinnoh series, had several memorable characteristics and skills inherent to him coming of as very enjoyable character when given screen time to interact with others(see Holly, Miltank maid girl from cafe, trainer with Ditto, arrogant girl which got on his nerves with Cherubi etc)or various times he risked his life to find special berries after Ash crushing defeat vs Paul in order to heal his Monferno. Taught Dawn about first aid healing her Pachirisu. Showed lot of adamant and asertive behavior toward Paul scolding him over careless treatment of Chimchar defending Ash trainng style while battling, getting focus on his interests or push other trainers and story to go forward playing role in it.
Problem in DP in contrast to other series Brock was not his character, but large discrepany in screen time arranged between main characters with Ash and DAwn getting more than needed. While leaving others in dust which reflected negatively on some rvals too(like Nando, Kenny, Ursula etc).
He shined in special with Cilan though showing top qualities and reason i like him taking lead in healing Pansage, helping Cilan and Jenny to work out plan in catching pokemon poacher later healing Gyarados climbing on his back showing him in impressive light. Not to mention having fun interactions with Cilan with both finding hemselves in awkward situations. Getting clues of talking about same person (Ash) but failing to figure it out or demonstrating his cooking abilities showing lot of energy.
If anything BW special featuring both Cilan and Brock showed how writers certainly can do better job with him making character appealing again(at least imo).
TLDR:Just like we as humans can sometimes fall under bad influence, deteriorate as persons and go of "right path" . In future being possible through someone guidance, understanding and help come to realization what we have done wrong improving as persons becoming better and worthwhile to society and ourselves again.
Same can be applied to anime characters writers have written for. Because "stale"(subjective to begin with) to some characters can become interesting again by fixing mistakes and refurbishing appeal they used to have among viewers through innovative writing and new subplots. "Old can become fresh", "boring can become interesting" and "pointless can become relevant again".
Its not impossible, never was. And it would be nice for a change to see writers investing more in characters proven to be popular and they created taking them in new, exciting directions. Building up on history, friendship and their own stories, rather than just adding new ones who end up replaced and forgotten as well. Forgetting about their previous creations, fun qualities and prospect of going forward enriching on personalities asnd their interests making fans sad and disappointed. Because of their favorites being abandoned and forgotten just like that(not a good thing at all imho).
