I'd rather have had May never leave, but anyway...
I say both of them should come back to make cameo appearances, but as for permanently rejoining the cast... Dawn took May's role as the coordinator of the group, and for multiple reasons there isn't really any reason for May to replace Dawn. The writers have also been treating May like she hasn't existed ever since she left, so her rejoining the cast would make things even more awkward. Plus, if the writers were willing to have May rejoin the cast, why did they get rid of her in the first place? Also, May said she would train on her own now. Her permanently joining with Ash
would be just as contradictory as her joining with Drew and/or Harley.
As for Misty, there isn't anyone who took her role away as a Water Pokemon Trainer, but why should she come back after being absent for 250+ episodes or so (not in a row of course)? I'd be annoyed if she just rejoined the cast just to be a side character (she can rejoin 250+ episodes later as a side coordinator but the coordinator of the cast, whichever one you want, can't be the main coordinator of the show?), so she should have to provide something new to the cast. (Brock hasn't provided anything new as a breeder, and look at what many people think of him.) But that brings about another question- what purpose would spring up for Misty to do that hasn't existed in 250+ episodes? And why does Misty have to do it- because she's Misty? Furthermore, Misty's not really going to be able to argue with Ash as much anymore since he's matured as a Trainer, and if that's one of the things people were looking forward to when they want her to come back, then... well... they'd be disappointed then.
Anyway, basically, getting Misty back in the cast is easier than handling her while she's on the cast, I think. And May got laid off and lost her job to Dawn, so her permanently joining wouldn't work. That's why I think they should just get cameos (*cough* also a certain other person should stay in the cast *cough*).
May Had More To Offer As A Character, Plush Shes Not Rehashed Or Jail Baitfull Like A Certain Other Character.
I think many people would disagree with this part here.