In the Tv toon for Pokemon was good for so many years, but so many of Ash's vrienden disappear after that region. Brock is the only exception to that because he traveled through Kanto, Johto, Hoenn, and Sinnoh. Ash´s travel pals always disappear they should all come back frequently throughout the toon when they seem needed.
Brock should come back in the Tv shows because he seems to know almost everything that's going on in the regions. Everyone needs Brock sometimes if their Pokemon are sick if they are in the woods alone without food. Brock traveled through 4 regions though and he has a job that he needs to do being a Pokemon doctor so the chances of him returning are very slim.
May and Max have an okay chance of coming back in Alola but it might just be buy video conference of for one episode. May could be some good entertainment with her being a coordinator and Max could tell u what moves a Pokemon can learn but it's the same for them as for Brock chances are very slim.
Cilan has no chance of coming back even though he is a good Pokemon connoisseur he has no chance because he wasn't as good a friend too Ash as everyone else was. Now I´m not saying that Cilan was not a good friend he just was not as good of a friend as everyone else Ash traveled with.
Clemont and Bonnie have a decent chance of coming back in the toon because Bonnie was alway making the funny scene and Clemont's inventions and cooking were with Ash all the way in the Kalos region. Clemont and Bonnie have a bigger chance at coming back in the Alola chapters, but probably won´t.
Serena has a good chance at coming back in Alola because there has already been pictures out of Ash and Serena in Alola. Serena was always a good friend too Ash when they were little kids at Professor Oak's summer camp and when Ash was training for his bug badge she reminded him of what he told her when they were young not too give up until it is over. Serena´s chance at Alola is the greatest so far but she is very likely to come back but probably for just one, two, of three episodes in the series.
Dawns chance at coming back is about as slim as Brock's but she could cause some major changes in the whole show. If Dawn was too come back she would probably be mad at Ash for not giving her a call then Ash would take 20 minuten to explain himself and run away from Dawn trying to hide because he didn't want Dawn to have her Piplup use rapid spin on him. Still Dawn´s chances are very slim.
Iris is the same as Cilan which means that Ash´s Unova Pokemon probably won't come back either. Iris was a good friend, but Ash and Iris weren't always on the same page and that is why she will not come back in the Alola chapter.
Misty has the same amount of chance of coming back as Serena because she seems like the person who would get there bike fried again just to make Ash take her with him again. Though it wouldn't happen that way she has about the same amount of chance at coming back as Serena.
Tracey won´t come back in Alola but he might make an appearance for Ash talking too Professor Oak he is Oak's assistant so he collects all the Pokemon that Ash sends back too the professor.
So even though some of Ash's vrienden might not make and appearance they will always be with him throughout his journey in his memories.