People change all the time careers in real life so it wouldnt be particularly strange if someone else beside Gary or Brock follow same path too.Wit Gary it kinda made sense to decide following his grandfather footsteps becoming fascinated with all kind of different pokemon species and their evolution realizing after Johto tournament what h wants to do with his life.
With Brock last minute decision of deciding to become doctor was just bad writing with no proper build up or foreshadowing happening prior to that.However he showed some interest for such thing earlier in DP and at end of AG so in a way becoming doctor can be viewed as step up from pokemon breeder with both careers having several things in common when it comes to taking care of pokemon,healing them and keeping in good shape.
Out of every past character i say biggest possibility of someone changing his goal would be Dawn.We already saw her showing quite a lot of interest in becoming pokemon stylist admiring Paris and her Lopunny.Since Dawn main ambition was to become top coordinator of one region like her mother Johanna and doesnt seem to have any aims of striving toward something more once she does everything she can in this area i could see her changing career deciding to become pokemon stylist following footsteps of her idol in that branch.Not to mention she has Buneary which would be suitable pokemon for someone who pursue this goal as we get to see with his evolved form.
With others chances are little smaller.With Misty as it was showed her dream is still to become water master taking over gym out of necessity and obligation(Not because she wanted)and for all we know seeing how she wasnt been showed for years she could likely went on her own adventure in meantime advancing ultimate dream(Like Wallace did who left gym leader position to learn new things and improve skills before becoming champion and on of best water trainers in world).
Sam with May,it was showed how sh still plans becoming top coordinator judging by her last appearance.
They switched Misty, Brock, and Gary's goals in a way to write them out of the series. Not to mention their initial goals were hardly focused on in the series anyway.
Although I get the suspicion Dawn should probably become a Pokestylist instead of a coordinator.
While some like Gary or Brock changed their goals,Misty never changed her dream.
She only returned back to gym when her sisters called her back out of obligation later on staying there because of sisters incompetence not having choice but to stay there since they didnt want to take care of it anymore.
We can see that in episodes like "A Date with Delcatty"where she complained about having to do everything by herself at end of episode with sad tone stating "how her sisters are of no use,having to stay there doing her best",or in her cameos like "A Real Cleffa Hanger" where she showed frustration of not having anyone to rely on when Daisy rushed her back not being able to leave place even for a moment.
Chronicles didnt showed any indication of Misty character giving up from water pokemon master thing or showed any indication of dropping it like Gary or Brock for example did.She always viewed gym as some kind of job deciding to stay there to help sisters out and because of not having substitute to leave it in its hands.If anything her complaining about sisters carefree attitude not jumping in to help her out,frustration with not having someone to rely on leaving gym in his hands(Like in Real Cleffa Hanger)and showing displeasure still being interested in learning everything she can about water types showed how she still hasnt given up on her original career of becoming water master with gym not bing hr dream.
Similar to how Brock had his dreams but didnt had a choice but to stay in gym taking care of it until his father returned being free to leave.
And as far as Misty goes, they had her become a full-time water-type gym leader... isn't that what being a "water pokemon master" is? How many times have we heard a gym leader being described as a master of their chosen type? Almost EVERY time!
Except its not,with gym leader thing not having anything to do with Misty goal.When it comes to becoming water master,this was stated to mean becoming strongest water trainer in world like E4 are who are best trainers out there specializing in one type being one of reasons she traveled not having any interest in gym.
Furthermore Misty herself stated for Lorelei to be her idol admiring hr skills and knowledge when using ice and water types dreaming to become that strong one day.
It was established in original series how carer of water master is similar to Ash goal of pokemon master requiring traveling,meet strong trainers and learn about pokemon in wild along with advancing it through tournaments/competitions.
We can see further confirmation to that during Whirl Cup when it was clearly showed how there exist tasks someone needs to win in order to become water master,with prof.Elm explaining connection between whole tournament and goal of becoming master of water types.Like when he mentioned how winner gains title "Alpha Omega of water types" being one of tasks water masters in past accomplished.
Just like coordinators have contests,Ash gyms,in Misty situation there existed tasks she needs to win in order to achieve what she strive for too.It just happens writers never expanded more on it.
It was also established in pokemon anime how there exist different meanings for one type masters.Like "Water pokemon master"for example being explained to be strongest water pokemon trainer in world with prestigious title being given to those who proven themselves worldwide among other water trainers,while with Iris goal it was showed how "dragon master" means very specific title which is given to those from dragon village who have proven themselves to be worthy of it by having immense knowledge and expertise about dragon types.
On top of that it was showed how E4 members are probably closest thing to one type masters with some so far being called as such like Lance being labeled as unbeatable "dragon master",Lorelei which was referred as master of ice types etc.Somehing which gym leaders arent and hasnt been referred as such
