I'm sorry, but I really fail to see how a person who started out hypocritically going on a Pokémon trainer journey even when she downright hated Pokémon is at all a good character, period, regardless of whether she's comparable to Serena or not. I'm no Serena fan, and I'm certainly not fond with how Amourshipping was handled by any stretch, but at least she had an actual reason for joining Ash even if she didn't have a good reason to go on a journey, at least she didn't outright HATE Pokémon, while May should NOT even be a trainer in the first place if her hatred of Pokémon in the beginning is anything to go by. And quite frankly, her relationship with Manaphy was soured the second she let Phantom abduct it and made absolutely no attempt at stopping him or even doing anything to save him in what was supposed to be her movie (That's essentially your kid there. Mothers would do ANYTHING in their power to save what is essentially their own child. If this were Misty in her position, you'd bet that she'd do everything she can to save Manaphy).
And as far as Dawn, she literally retreaded May's goal, probably the only real difference is that unlike May, Dawn actually wanted to be a coordinator from the very start, and overall from watching the first few episode, I tried to get into Dawn, but just couldn't, with the Buneary episode eventually killing off any likeability I might have found in Dawn. And even if I DID like May and Dawn, especially to the degree that you do, I still wouldn't see any reason for them to return largely because there aren't any contests in Alola (in fact, of May, Dawn, and Serena, only May I could plausibly see possibly visiting Alola, and that's only because she most likely would have already taken care of Johto and Sinnoh by that time anyways, as we already saw her trek in Hoenn and Kanto, and thus she probably won't be impacted by the regional limit nearly as much.).
And quite frankly, like I said earlier, I doubt Serena would have much chance to return, especially when her returning would actually cause some rather negative implications about her possibly ditching her goal so she can be with Ash in a romantic manner.
And I'm not going to speak for Lillie since, like Serena, I don't have much of a clue on what her character is like, as unlike AG or DP, I've never watched even a second of Best Wishes, Sun and Moon, or XY.