Considering that Misty never reappeared after her one-off stint(s) during Hoenn, it should have been obvious that she would never reappear for Sinnoh or any following region's arc. We all have our characters that we really like, but sometimes we have to accept it if things don't go our way (especially when it comes to characters not returning ever again after a long absence). We also have our fanon thoughts on how things should have gone on our favorite franchises, but it's not the best idea to claim over and over that such things should become official canon.
Problem is, she actually was forced to put off her goal and become a Team Rocket-esque punching bag by becoming a Gym Leader, despite the fact that she had a Water Pokémon Master goal that was made blatant at least since the start of Johto, arguably as early as even Kanto. Even if I hated Misty with an utter passion, I still would have been dumbfounded at the sheer idiocy of their removal of her and the way they did so.
The other members of the main cast at least were given the promise of pursuing their goals when they left, and in a few cases even accomplished them, and Brock at least chose of his own free will to change his goal from breeder to doctor (granted, I'm still very disappointed that he didn't take over the gym from his own family after they twice demonstrated their utter incompetence, enough to make even Misty's sisters seem like hypercompetent and skilled Gym Leaders by comparison, but that's another topic for another day).
And let me ask you this: Suppose May, for example, decided she wanted to go to Johto, only she then gets an emergency call from Caroline stating that Norman either got into a very serious accident or otherwise came down with some illness, so May needs to come home ASAP to take care of running Petalburg Gym, at least until the time Norman gets better, and thus she is forced to put off her goal relating to becoming Top Coordinator to do so and was very unhappy with the result. Would you think that was even a remotely good end for her, ignoring your own personal feelings about her for just a moment? That's exactly what I'm trying to convey regarding Misty's departure, and precisely WHY she needs to come back above all others (the only character who might have it worse off than Misty is Max due to the writers' stupid insistence on keeping Ash 10 and thus pretty much forcing Ash to break his promise about battling Max in the future.). The fact that AG just went through depicting the Gym Leaders as pansies on par with Team Rocket in Ash Catches a Pokémon at best doesn't help matters either. That's why Misty needs to return, if not to the main cast, then at least as a cameo that ends with her explicitly NOT returning to Cerulean Gym at all, but rather continuing her journey.
I agree. She certainly seems too strong and capable to be a Gym Leader for rookies. I vaguely remember her Water Pokemon Master goal. But I get what you're saying. But I think it puts her in a great position to mentor up and coming Pokemon trainers. Brock from Pokemon Origins comes to mind. Plus, it's a job.
Don't think it necessarily ties her down as other Gym Leaders travel, train, and possibly research. Who knows, the writers may surprise us by making her a Gym Leader elsewhere or holding a different position. I'd be interested if they covered a Gym Leader's life in the anime sometime or at least as part of a offshoot series. Like Pokemon Generations now, except Pokemon world building with a focus on Gym Leaders and more.
Heck, if they could do a random episode with Misty doing her own thing and running into Ash accidentally. Then seeing her character growth and also as a water Pokemon trainer.
Yes, please.
Yes, fully agreed with showcasing the Gym Leader's capabilities. That's one of the things I hated about AG and BW, as they generally had Ash's rookie team basically tear through the Hoenn and Unova Gym Leaders (and the fact that Ash went full retard mode in the latter region doesn't help matters either). I get that the Gym Leaders are not supposed to be as strong as the Elite 4 at their current position, let alone the Champion, but that did NOT mean they needed to be so weak that their defeat many times were comparable to Team Rocket's defeat in Ash Catches a Pokémon (I think you remember what I'm referring to, where a nearly dead [for reasons completely unrelated to Team Rocket], completely inexperienced Caterpie managed to own Team Rocket their collective butts and tear through them like tissue paper, and in fact that particular defeat was notorious enough that it led to not only the fans, but even the in-universe characters, to mock them as being weak, with it even being something of a running joke that Misty and Brock whenever Ash brags about his accomplishments often tells him that beating Team Rocket's not even remotely close enough to proving Ash has any skills, which occurred as late as Johto). In fact, I still maintain that the Johto Gym Leaders were probably the best of the Gym Leaders so far (yes, better than even the Kanto Gym Leaders), as not only are they appropriately shown as being, while not Elite 4 level, definitely forces to be reckoned with especially against rookies (even if against more experienced vets they can be brought down a notch), but Ash actually won his battles fair and square. Second best would be Kanto, since while they are definitely forces to be reckoned with against Ash especially given it was his first time, half of the badges he won for reasons not related to winning (I personally think Lt. Surge was one of the better Gym Leaders, though really, all the Gym Leaders were reasonably competent in Johto, even Team Rocket when they had their stint as Gym Leaders. In fact, the closest I can get to truly incompetent Gym Leaders were Misty's sisters, and even they at least were implied to have held the Gym quite decently until the trainers from Pallet passed by the area). DP and XY did showcase the Gym Leaders being skilled, but unfortunately, they did it in the opposite extreme, where they ended up making one of the vets on Ash's team, Pikachu, look bad by comparison (which is even worse since Pikachu was verified to have beaten Regice single-handedly, meaning he should not need to lose to Gym Leaders or, heck, Rookie Pokémon even).
And I definitely agree regarding Misty returning, they definitely should have her come back, if for no other reason than to ensure she doesn't have to run the gym again.