True about Dawn and Brock taking precedence since they were recently replaced characters not getting regional cameos yet but after them i could see Misty coming back at some point.
Dawn might have some chance to return as main though if you ask me Misty also might have chance to return because of being part of original trio which made pokemon popular in first place becoming iconic and worldwide recognized.Not to mention from story standpoint writers developed her alot less than they had other girls having more room left for further progress having loads of unfinished things on which they could expand.
What im trying to say is that while not big(infact its low)Misty also has some small chance of coming back as main character for continuity purposes relieving spirit of older days being recognizable and very popular character in case writers want to try going beyond target audience with purpose of attracting different kind of demographic toward this show(older fanbase)while still keeping new one.
Especially if Kanto remakes get made again having marketing value in that case along with being possible that writers might reunite Ash,Misty and Brock for last season to finish anime with group which started it to emphasize more on show going back to its roots providing better sense of closure.
Not to mention demand for Misty return to main cast is huge than i noticed to be situation with any other character so far.
It depends in what direction writers want to go.They could always bring her for some water tournament or to advance her goal.Seeing how Unova is isolated region making hard to obtain its pokemon in older regions could also serve as plot-line to have Misty travel there.
Having Misty meet Iris would also make a lot of sense.Because of Iris connections with 8th gym and Misty being gym leader this characters aside from sharing some personality traits have things in common being beneficial to have Misty back and help Iris about possible gym issues to overcome(like May did with Dawn helping her to regain self-esteem).
Writers already showed with return of Gary,.Jessibelle or Jasmine in DP how unorthodox and unpredictable they can be when it comes to older characters not mattering much to them for how long character was absent if thy decide to do something with it and make relevant to current plot.If someone so minor could return after such a long time becoming relevant again there is no reason to believe how someone more popular and important who is to say Misty,Brock or anyone else cant as well.ESpecially when there are still left loose ends which werent tied up with writers having excuses(reasons) to bring her back having several unfinished things in story.
However seeing how Unova is portrayed as very far and unknown region not having much connections with older regions as well having lack of references with no older pokemon(except Pikachu)being present its hard to say if Misty will return for it or not.
However if there is filler arc like Battle Frontier was for Hoenn Misty chances would definitely increase though seeing how writers often like reusing older characters and pokemon for it.
In nutshell even if they dont appear after BW all bets for Misty and other characters return are still open.As for return as main its not very likely though but some small chance of happening there exists for several reasons i mentioned in some other thread.