This is gonna be longer post, but i feel to truly understand why this particular companion is my favorite and would like to see it return saying this much was needed.
Everyone have their own favorite, character they grew attached to, were able to affiliate themelves and symphatize for their flaws, personality traits, backstory ,effort put in accomplishing outlined desires etc.
For me that character with who i could relate myself to and experience most enjoyment, quality thought complexity and interesting plots making me excited and interested what would come out of it would be Misty.
Reason i liked/like Misty the most is because of strong, outspoken personality having many sides and quirks to her bringing lot of humor, drama and endearment in cast. Because of touching past having to live in sister shadow being constantly degraded having to work hard for everything she accomplished. Because of unique niche of being one type trainer striving to become best with water type, her pokemon and strong sense of presence managing to remain active despite not having fully fleshed out story like May and Dawn. I also liked her attitude standing up for weak, not letting others to put her down and often taking initiative to move things forward giving to others motivation to never give up from their dreams.
Imo character value shouldn't be measured by how much he does, but by how valuable and important things he brought to show were and how much of impact he left in general. And way i see it Misty contributed alot. She brought lot of sharp and witty humor, flamboyant dynamics and comedy in cast. Helped Ash to grow as trainer ad person helping him abut basics and making sure to stay out of trouble. Entered various races, tournaments and contests, often influenced story by saving other lives, getting group out of danger(such as from sunk ship convincing berserk pokemon to stop with destruction, baited Ash out of jail, took care of Ash, Brock or Tracey when they were poisoned or sick etc), came up with various jokes and solutions to clues solving mystery behind kidnapped pokemon, helped others which had issues with older siblings and friends to fight for themselves ,preventing natural disasters caused by pokemon or human factor etc.
She had plots and story too. Like complex of inferiority imposed on her by sisters, had goal of becoming water master, strive to learn as much as she can about pokemon types and some day meet and catch legendary water type. Had fears which posed obstacle on her way like Gyarados and bugs along with often giving us insight in her fears ans struggles she went through getting some valuable growth(like overcoming childhood fear, improving as trainer, earning sisters respect and becoming more confident and levelheaded valuing her friendship with others.
Personally i find Misty is great character and most captivating, emotional companion Ash had which never got chance to be used to full potential with lot of material about story ending unused/unfinished. And if by some miracle she ever returns again whether through appearance or something more i would welcome it.
Because at end of the day who are we without past, without traits which define us and with no legacy we could leave for futue generations behind ourselves?
Person without identity.
Same applies to pokemon anime. With older characters being ones which builded this show history, played role in developing its stoiryline and left impact on it making it more popular, accesible and recognized worldwide.
Older characters coming back is requied if you want to have quality based storyline, connection to past and events which preceded to what is set to happen in future. Increasing continuity, make story more cohesive and better connected. Thus acknowledging on charisma and influence Ash past friends left on pokemon, strong tight bonds and friendship which should exist between them and main hero. While giving to viewers updates, chance to enjoy in their favorites, their silly antics, fluid characterization and passionate traits deserving that their unfinished story, dreams, goals they work toward to, unsolved isues and personality get explored more and remain relevant in some way.
And out of all replaced Ash companions i want Misty to come back the most. We havent sen her for very long time and unexpected return would be a nice twist and change of things bringing something unexpected yet new on table.
It would be interesting to see how she would interact with new characters adding that extra flavor to character dynamic making it better.Purpose could be to advance her goal and since many things are left unfinished about her past,ambitions,goal of water pokemon master etc it would be interesting to see her developed more. Like traveling with purpose of advancing water master dream by expanding aside from Whirl Cup,introduce her rival,develop her pokemon etc.
Having her go on journey to become Lorelei or some other powerful trainer apprentice and eventually become E4 of water(basically achievmnt of her dream) would be cool too, accompanying Ash and group for onme last time before she gets to destination and achirves something big enough that we could say how she came closer toward her dreams.
Getting some closure she never really was able to receive.
Because i miss original pokemon girl wanting to see her again, see how much she changed, enjoy in her various antics, funny outbursts and lively quirks and see some follow up on her well at this point unresolved story. More development from Misty playing some valuable role in pokemon again after so much time wouldn't hurt especially with so many potential and innovative directions to take her forward. Right?
It would kind of unexpected, fresh twist to story and i imagine she could lift up atmosphere in main cast through her adamant, curious, hotheaded nature finding common language with new incomers and revive friendship with Ash. Now that both are more mature not seeing each other for very long time reviving on that sarcasm, flare, funny quips and respect, admiration they holded toward each other being great friends.
Its been 9 years and good old fiery, sweet and passionate redhead deserves to come back.;)