In event of May deserving to get special i definitely agree with everyone else and if chronicles didn't stopped airing im sure May would get some coverage on her Johto adventures. Her leaving after Wallace Cup left many things open and unanswered about her future growth as coordinator, how far she came in Johto Grand Festival, to what region she headed afterwards etc.
It definitely had potential to make interesting storyline out of it, and seeing some of her rivals again in action like Drew and Harley would be highly appreciated too. Judging by huge improvement she made as coordinator when returning in DP having already 3 ribbons and bunch of evolved pokemon like Venusaur, Wartortle and Glaceon i wouldn't be surprised if she even went all the way winning GF without us even knowing. And we will probably be left in dark on that regard unfortunately, unless she appears again.
Which may or may not happen in case Ruby and Sapphire remakes ever happen, though it would be stupid to leave May out of it. Since she is female heroine of third generation and best representative of this games. Due to popularity and prominent status she secured for herself during her reign in anime.
Nevertheless having pokemon that have access to mega upgrade making her perfect cadidate for third gen remake and XY games promotion.
But honestly, character which in my opinion should get priority in coming back before anyone else in Kalos(after Iris and Cilan get their follow up of course) should be Misty.
Its been way too long since she was showed in person (last one was in 2005 in regular anime episodes, passing over 410 episodes which is well unfair). Its at least to me saddening to see how one of most realistic and fun characters, original girl which helped form pokemon and spread its popularity in first place being treated with such disrespect this days.
Leaving her dreams of becoming water pokemon master, learn as much as she can making name for herself among other water experts reaching E4 strength, overcome her unresolved flaws and fears(im certain bugphbia would get in way of accepting Surskit, or having lot to learn in extracting pokemon true potential on surface) unfinished and left hanging in air with no signs or direction of in what way her story will continue ever being provided. Rotting at gym like she never existed or her plots mattered.
I mean since XY region is based on France would be ideal region for Misty to travel being fond of romance and love opening ground to want to go and explore new pokemon and culture there.
I would appreciate to see how much she changed in meantime as person and trainer, what new pokemon she may have, did she improved in her dreams of becoming best water trainer and whatever changes might have affected her on physical and psychological base. I know i sound like broken record but presence of E4 Siebald, having Gyarados which could open gates toward mega evolution, flexibility of her role not depending on game assets to come back and advance her story of becoming water master but main plot too through anime only events(like water based tournaments or entering Battle Chateau if there is more in future to prove her woirth and advance forward) are all interesting and feasible ways to have her back.
Making it easier to return and advance story than May and Dawn(for them to truly shine they kinda do need contests).
And her defiant, tough nature mixed with feminine aspect having interest for girly things and fashion too could click well with Serena helping in becoming more assertive, how to stand up for herself and find purpose, something she wants to do and accomplish(goal). Helping best in serving as push for Serena to become more independent and resolute in her future actions.
Shame it most likely wont happen though, because i would be interested in her forming relationship with blonde trio and restore on friendship with Ash lifting up group dynamic being refreshing for change to see more experienced, mature girl growing forward building up on story and new plots.
Oh well, maybe in unknown future that day will come. Hope dies last, right?