To me Brock was great character and one of most iconic and distinct protagonists of pokemon series. I liked his wisdom, maturity, lot of knowledge he brimmed with. All enveloped with assertive and adamant side of his making sure that Ash and co dont run foolishly in danger , scolding those who wanted to use pokemon for selfish purposes like Damian with abandoned Charmander or Paul with Chimchar, or when someone would get in his way of getting girl. His agitated and boiling side was especially well depicted in relationship with his crazy family serving as nice contrast to usually cool and collected attitude. Being ashamed and irritated when his mother turned Pewter pride in circus(referring to gym of course).
Brock also showed lot of compasion side, care and thoughtful side of his. Best displayed in care of lot of pokemon he encountered on joureys applying his knowledge as breeder such as sick Bagon, poisoned Sharpedo not wanting to leave him spending night on beach, defending Solrock who got accused of drought happening in vilage ruining all crops. Taking care of sick Pachirisu from Dawn teaching her basics of medical aid, when Nuzleaf got injured and many others.
He was like older brother to Ash developing strong, full of understanding and interchangeable encouragement friendship existing between them often offering help to be like sparing partner when training Growyle, Torkoal, his Chimchar helping about blaze issues etc. Involving to risk his life on several ocasions to find specific berry helping to heal his pokemon (this was especially evidenced with Monferno when losing harshly to Paul).
He also had very good relationship with main girls. Not as much with Dawn but with Misty Brock established close almost like brother-sister relationship often joking, talking about Asd loses and reckless side and how to prevent him from making fool of himelf or giving comfort. Talking about romance, their own dreams, sharting experiences they had as gym leader and sometime getting in clash with interactions being playful and full of energy. With May interacting was pretty good too, maybe not as nuanced like with Misty but Brock quite often accompanied May on shopping streaks, May showing interest in his cooking and food along with Brock giving her tips and encouragement for her contest battles(such as recommending assist to be used from Skitty).,
Brock didnt got much progression on his breeder career but on good part this was hindered by passivity of his own ambition being type of career focused more on taking care of pokemon and grooming them, rather than battling, training, entering tournaments etc providing issue in finding ways to moire actively demonstrate progression.
Second part of reason came from inherent imitations concept of breeding had on its own, with there existing restriction on how far writers can go with exploring idea of pokemon reproduction in kids TV show like pokemon.
However regardless of this i personally liked Brock, his antics, kinky interesting personality, huge amount of knowledge and balanced, reassuring atmosphere his character brought to main cast. Having several remarkable times where he truly shined as character in OS, AG , even ever so loathed Sinnoh , helped move show and characters forward being important backbone on which others could rely keeping things on hold and preventing from everything falling apart giving shoulder to cry, confine yourself and help to get back on feet again. Which had its merits in my opinion.
I wouldn't be against Brock coming back either, because every traveling companion who was in main cast and important part of pokemon history, Ash life helping to build its plot and popularity deserves to remain acknowledged and relevant.
With no one deserving to stay buried forgotten and erased from face of existence depriving fans of update, how far someone came in progressing toward achievement of dreams, what kind of new fresh flavor to their personality new experiences, struggles and encounter with new people of screen has left on them changing because of that and witnessing if strong close friendship with Ash was able to maintain itself despite long separatioin. Giving fans of Brock or anyone for that matter treat, follow up and chance to enjoy i their favorite characer going in undiscovered, exciting adventures playing importance in future again.
Afterall Brock is traveling to gain new knowledge, discover new species and meeting doctors who could teach him important lessons as part of stufying before passing tests. Unova special featuring Cilan proved that when Brock was on foot meeting connoisseur in same region.
So as traveling doctor in training nothing is stopping Brock from meeting up with Ash and even joning on new adenture.
However if im going to be perfectly honest whle i liked Brock i was much more displeased personally when Misty got replaced. Because she was one of best, most iconic characters this series had being deprived of that humor, clash, sarcasm, energy and identity her strong personality brought to it. Leaving lot of potential untapped , completely open story and so mny missed opportunities to do more refined growth with her regarding inner struggles, uncleared feelings for others and water master career striving to become strong as E4 members not even knowing from where such determination and desire came from.
Feeling how in case of coming back Misty would bring more unpredictabilty, diversified characterization, novelty and content to story than Brock could.
p.s. I also feel moral obligation to note how whether Brock became stale or not is purely matter of opinion and how you personally felt about character. Because if you didnt grew sick of Brock still being entertained by his behavior, interests, pokemon, goal and influence he left on other characters and plot. That absolutely fine, just like its fine if you grew tired of him not being able tp find anything positive or good about Brock presence in pokemon series anymore.
There is really no universal fact or opinion for this. Hence why term "stale", "boring", "useless" etc are completely subjective varying from person to person.
Its one day earlier, but i want to wish everyone Merry Christmas and happy holidays. All the best everyone. :)