Welcome Auaria~!
Yea, we did lose something when Misty was replaced with May. I wasn't mad with the decision that she had became Gym Leader. In a way, it's good for her since she can hone her skills at becoming a better Pokemon Trainer. But, what troubles me is, how do you become a Water Pokemon Master? You would think you would have to become like Wallace. And he's a Top Coordinator and the Pokemon Champion of Hoenn. Maybe you would have to be one with Water Pokemon like being a Dragon Master? It's a shame that it was never explained in the anime. A perfect time to explain it would have been when the gang had arrived at the Whirl Islands and was about to enter the Whirl Cup.
I believe main reason why many were disappointed with Misty exit was because they didn't really build her up for anything, leaving in half of development without providing conclusion for any plots which surrounded her.
She had unique story and goal of becoming water master with fresh material waiting to be explored, unique pokemon and established popularity existing potential and prospect to flourish but instead they canned her without much closure leaving much more to be desired. Which was poor and bad move imo.
As far as her goal goes judging by what we saw in original series is similar to Ash requiring traveling,meeting strong trainers,exploring new regions and learn about pokemon in wild along with advancing it through tournaments.
I mean for same reason Whirl Cup was introduced as one of such competitions where you can gain more experience,make step forward and prove yourself to others being directly connected/stated with water master quest.. As evidenced through dialogue from prof. Elm they met when registering and sea priestess Maya title of water master means becoming water pokemon expert which has power to understand sea creatures and use them to full potential,being recognized and respected among all water trainers worldwide.
And just when writers finally started to dive deeper in whole thing by expanding more on whole concept , soon enough they removed Misty from main cast stopping opportunity to explore more on her quest and do follow up through other challenges and tasks she would need to pass shading light on whole thing. See her grow forward as trainer finally forming stable, successful team and being developed to certain degree as more competent, confident character which fulfilled criteria required to go on next level.
Shame because Hoenn which was ruched with water or Sinnoh which introduced cool new pokemon and water specialists offered so many ways to develop Misty forward and add more substance to her story.
Becoming water master likely requires to achieve E4 level of skill since they are strongest one type specialists in world being "best mono trainers in world" drawing parallel with Misty desire of becoming strongest water trainer in world. Connection which revealed to exist even more when they met Lorelei back in Orange Islands wit Misty admiring her battling style ad skill in usage of water pokemon being to her idol.
Assuming Misty still wants to become water master and achieve more than being gym leader i imagine at some point she will leave gym to go on venture to learn things you cant at gym, enter tasks like Whirl Cup showed and become true expert in that field by battling other, more challenging trainers and learn more about water pokemon in wild.
Something which Wallace as you mentioned did leaving his position to advance as trainer and improve skills resulting in not only becoming Hoenn champion,and master in contests but also one of top trainers which use water type in world.
But, I loved the new ending for Da! Seeing Misty in BW's animation was truly amazing and I cannot wait to see her again. Maybe we'll see her when Ash returns home before he heads out on his next adventure like she did when he returned from Hoenn.
I have still some small hopes, but knowing writers its best to keep our expectancy in check not being first time that they disappointed us.
Its sad really, they opted to give us so much "fanservice" ever since BW2 began but didn't include one of most popular ad iconic characters in this show for who people have waited for years to see her back as evidenced through various petitions, campaigns and visible demand on internet.
Either writers are ignorant or don't know to recognize chance from which they could increase revenues and bring more fans toward this show.
I can only imagine how much hype and interest there would be sparked among people if they finally brought Misty back in person, or even went as far bringing her for season or two which would lift up dynamic in cast with her strong characterization and bring dash of fresh air by offering something unpredictable and unseen in sense of following aspiring water trainer advancing and going on bigger, more demanding challenges building on her quest of mastering water type.
I believe reasoning behind her return would be more original and unpredictable than its case with other companions(like coordinators for example from who we know what to expect), since Misty story and dreams offer several directions through which writers could create sequel and take her forward not having set up paradigm and procedure needs to be followed.
Something which has yet to be revealed with anime exclusive carers.
Going on quest to become E4 member or renowned water specialist could definitely provide foundation to make plausible continuation of Misty story with purpose of coming closer toward her dreams while also creating way to fit her within main show again.
Too bad writers can't recognize good opportunity when it arises rather choosing to constantly add new companions which end up canned unfinished anyway, instead of investing more in previous ones who have potential to offer more and go through more meaningful evolution than newbies could.
