Back on topic: Out of the four of them, I would actually want to see May the most, simply because we never found anything out about the Johto Grand Festival, she was actually doing something, unlike Misty, but I like I said just before, I would prefer to see all of them.
Im not sure if i get what you mean.
How exactly Misty wasnt doing anything?
She was supporting character who acted like Ash coach/mentor.She wasnt co-star but she did things providing humor and great chemistry between group,had several notable achievements(like winning Princess festival which was some sort of early contest,Seaking competition,being among top 8 in Whirl Cup, etc) and aside from battling did many other things which influenced plot in one way or another.
Like saving people lives,towns,taking care of those she cared about,helping to beat TR organization,giving moral support to her friends teaching people to never give up hope believing in themselves.
She was one along with Brock who influenced Ash a lot helping him to grow in trainer he became later giving identity to this anime and group as a whole.
She had her own goal of becoming water pokemon master too,and just because she wasnt doing contests collecting ribbons doesnt equal doing nothing.
However while i would rather prefer that Misty returns before anyone else being interested in seeing what new pokemon she has,what happened with her in mean time,see her doing some step forward with her goal perhaps(like entering some water tournament) having lot of untapped potential and unfinished thing in story i agree how May deserves to be seen again as well.
That being said its shame that we never get to find out how May went at Grand Festival in Johto with writers never giving fans some coverage.This could also be opportunity to see some of her rivals and how much things changed since last time we saw them.
Judging by Mays performance in Wallace Cup and her evolved team she showed big improvement as coordinator in rather short time span so i wouldnt be surprised if she managed to win Johto GF having what it takes.