There was confirmation recently that The SuMo anime isn't a sequel to the XY&Z series so it's highly unlikely we'll see her again. I'd love to see her again tho XD
Source:
http://satoshipediaeng.altervista.o...haracters-are-completely-new-ash-and-pikachu/
It's funny because you said "Serena has nothing to do in Alola because it doesn't help for her goal" but it's the same for Misty , she is a gym leader so there's no way she'll leave her gym to go to see Ash and I don't understand why she would deserve a return more than Iris or Serena since she already returned twice.
Now you said it yourself Brock and Serena are the most requested characters for a return by the fans so one of the two will be the priority IF the writers want an old character back BUT I don't see any old characters coming back with the new art style that's why I think it could happen in gen 8 because there's no way they'll keep this art style in gen 8 seeing how the fans have mixed feelings about it (don't kill me SM fanboys it's just my opinion and a pure theory). Anyway, for me there's no point to talk about Serena now , let's at least wait until 2018 to talk about an old character returning because it's always around 1 year and half after the begining of a saga that a cameo happened before XY broke the rule :
AG started in late 2002 : Misty's cameo -> September 2003 (1 year)
DP started in late 2006 : May's cameo -> April 2008 (1 year and half)
BW started in late 2010 : Dawn's cameo -> June 2012 (almost 2 years)
So yeah , no point to talk about Serena now except starting stupid arguments because there's always one guy who'll hate on her and one guy who'll love her , almost impossible to be neutral with this character in the west.
It's possible that she will appear but I doubt it. Iris didn't appear in XY so I think Serena will not aswell in SM.
But I could be wrong since she is very popular in Japan.
Maybe she is. Personally, I think having her return even temporarily for Alola is as bad of an idea as it was when they had Dawn return for Best Wishes, mostly because she literally has no business regarding her own goal of Pok?mon Coordinator right now in terms of being there due to Alola not having contests at all (heck, I'm not particularly happy that Ash's goal is being squandered at Alola in what is essentially an Emperor's New School ripoff, all because the idiot writers decided to maintain the status quo at the most inappropriate time in Kalos, so don't think I'm singling out Serena or Dawn for this aspect.). I may not like May at all, but at least she had an appropriate reason for returning for Sinnoh due to the Wallace Cup, so I could tolerate her returning. Even Misty's return in Hoenn related directly to her Togepi at least.
Shame Iris didn't get a return for XY, since, while obviously not to the extent of Misty since at least Iris actually IS pursuing her goal right now, it would have been nice to see how far she came with her goal.
Iris didn't return because she isn't popular in Japan. I also wanted to see how she improved and if her Gible evolved (since Ash's probably will never).
Misty had her time, she appeared in Pok?mon Chronicles and Advenced series (twice), even though I wouldn't mind to see her return.
The thing is Serena has the priority to appear since she was the last to leave Ash and had less episodes than Misty. Since I doubt that Serena will come back I doubt even more that Misty will.To be honest, I wouldn't even say Misty had her time, since unlike the other former cast members save for maybe Max (who arguably had it even WORSE than her since, thanks to the bit about keeping Ash 10 even when it makes absolutely no sense, he can't even hope to become a trainer, let alone face Ash in battle as promised when he left), she wasn't even allowed to either continue her goal or even choose of her own free will a new goal. She instead was forced by her sisters to change her goal to something she didn't even like at all. May, Dawn, Iris, Serena, Gary, Brock, Tracey, Cecil, and Clemont at least had the ability to continue pursuing their goals, maybe even actually succeed in their goals by the time they left in the case of Tracey (who's goal entailed meeting Professor Oak), or barring that, at least changing their goals of their own free will (such being the case for Brock, Gary, and Serena). Misty never got that, she never got ANY closure for her goal, nor any opportunity to advance in it, and to make matters worse, she didn't even have any choice in the matter. Now, if Misty had left to pursue her goal way back in Johto, or certainly at the end of her last major appearance in AG, pursued another water type thing, gone her own direction without being forced into the Gym, THEN I could say she had her time.
So far as Iris's popularity, or lack thereof, being the reason she didn't get a return, as much as I want to say that's the case, I doubt it. And that's mostly because even Tracey and Max got at least one cameo when they left (and in fact, Tracey ironically is the only former cast member who we can guarantee to get another appearance due to being Professor Oak's aide), and they were very unpopular as characters, arguably giving Iris a run for her money. Not to mention May was apparently fairly popular, yet neither she nor her brother even got Hosos (not to mention Serena didn't get a Hoso either, something even Iris got, and the latter, as you pointed out, was unpopular).
The thing is Serena has the priority to appear since she was the last to leave Ash and had less episodes than Misty. Since I doubt that Serena will come back I doubt even more that Misty will.
She and Misty, not to mention Brock, have far more of a chance to appear than Iris, Dawn, May, Max, Clemont, Dent, Bonnie, and Tracey do, if that poll regarding who they want to return is of any indication.