Having first group reunited for one last adventure before grand finale would be nice way to wrap up long lasting show like pokemon with things coming full circle.
This would also provide opportunity to see writers tying up some loose ends going for them being given some character development.I feel it would be more refreshing if we would see older companions updated developing those who already have potential to grow more . With Misty half of things going for her were left open giving writers room to do many fresh and interesting things now that they improved.
It would be also interesting for a change to see how would Ash interact with older and more mature friends nowadays with Misty and Brock already knowing him for a long time. With new companions we can't experience such dynamic since they are just starting to get know Ash and each other.
Not just with them, but with about any older group really.
Misty never said how "she likes being at gym and considers this her new calling" in AG appearances.Misty's sisters have been back as early as the second Misty special, which took place in very early Hoenn. They simply have no interest in being Gym leaders anymore, and Misty likes staying at the Gym and considers herself the leader now according to her dialogue in her AG appearances. There's no reason to ever expect her to start traveling again, especially not with Ash. And it hasn't been, "5 seasons," its been 10 ENTIRE years.
She returned to gym only because she was left without a choice at end of Johto and later through chronicles it was revealed how main reason behind running gym is out of necessity due to incompetent sisters, not having anyone to rely on and leave gym to his hands.
Unlike its case with Brock or Gary who blatantly changed their carers with Misty writers left things unclear not knowing if she still wants to become water master or not. Judging by reaction in "Gotta Catch Ya Later" and in chronicles they made it rather obvious how she never gave up from her original goal.
Personally i agree with above poster about Misty coming back to cast for awhile(for season or two). That would be certainly unexpected twist and interesting to see, and since many gym leaders like Wallace, Fantina, Cilan etc travel to advance their skills and since Misty wanted to become best water trainer it would make sense if she left on journey, to improve and develop her skills. It would be intriguing to see her go on journey advancing career of becoming water master entering water based events and catching new pokemon bringing that certain charm and fun dynamic she brought to cast. Or we could see her on venture of becoming E4(equivalent to best water trainer basically)trying to pass tests to prove herself to evaluators.
If her sisters become more responsible, or someone else is find as substitute i could see Misty follow Wallace and many other trainers footsteps.
I would definitely give her chance in main series again, if by some miracle writers ever return her.