After Unova finish Ash will most likely as usual stop at Pallet town for half or whole episode to get new clothes and find out about new region(generation 6) leaving all pokemon behind except Pikachu starting another rinse and repeat quest.
Considering how writers aren't very keen on keeping characters chances for Iris and even Cilan stay are relatively small. And reuse of older previous companions hasn't happened ever since anime earčyx days when Brock returned to main cast after OI hiatus.
Sadly most likely option judging by past patterns is in Ash meeting another two inexperienced characters accompanying him for four years joining afterwards rest of crew of long forgotten, abandoned and unfinished ex main characters which used to be part of pokemon in its better days.
That being said i believe constant cast cycling is one of biggest mistakes pokemon is doing right now eroding foundations on which show was build destroying continuity and anime history. I don't see any point in constantly adding new companions when there exists potential in older ones to do more and be further developed, just starting all over again with someone else leaving in half of its growth anyway before he would even get chance to go through more meaningful growth.
It discourage people from investing in their favorites too when its already known from start how they will end up forever abandoned and unfinished after departure anyway, and its sad to see so many great characters we grew to like ending abandoned after they leave cast never to be seen again.
Writers should look up to other anime like One Piece, Inazuma Eleven Go or Bakugan. They could learn a lot from them and how to keep older characters relevant, when adding new ones.
In long running show like pokemon innovations and unpredictable surprises are key in keeping anime fresh and exciting, and return of older characters would ultimately make anime attracting to wider group of people.
Current approach of sweeping everything under the rug constantly erasing past in long run isn't sustainable tactic through which pokemon series can continue to last, so at some point in future producers should think of possibility to apply more conservative approach through which loyalty toward anime and certain brand could be made maintaining people interest.
Something which you can't establish with excessive cast changes.
Going by this answer has already been provided,but sometimes writers tend to change their minds.Like they did with Brock case in OI for example.
So nothing should be viewed as "set in stone" if you ask me because if writers regain interest in past character wanting to do something significant with him, in theory initial decision can be withdrawn bringing back past companion in future providing his presence could help advance current story.
Eiother way i fully agree with your idea.
At very least writers should take in account concept of character rotation(reuse of some older characters in future).
That way we would still get new characters ,but at same time fans would be given opportunity to see their favorite characters from past in action again,being updated,refreshed and developed more taking them in new/undiscovered directions.
I think it would be interesting to see for instance May developing new style as coordinator,do steps forward toward her goal and develop new pokemon and potential rivals.
It would be also interesting to see Misty being developed with many things being left unexplored.Like expanding on her goal of water master,do more with pokemon,reveal more about past and open potentially new subplots to enrich here story and do continuation..Trying to become some powerful water trainer apprentice(like Lorelei or Wallace)with purpose of becoming E4 is one of many other available option to take her in new direction and do significant growth too.
Same with Max for example,seeing him getting first pokemon and starting out journey would be more than welcomed to see.
This may seem strange but many people who watched AG,DP or best wishes wouldn't mind if original trio gets reunited from what i noticed.Sure they would rather like their favorite groups to see light of the day but if first group writers revive most of them would be either neutral or would welcome it.
Very few would be bursting out of anger with pokemon fanbase usually being open minded once writers something introduce in show.
However even if by some miracle original trio is reunited for last adventure prior to en having things come full circle, other companions which used to travel with Ash should afterwards receive arcs wrapping up their loose ends in stories, and getting some recognition for contribution they made to this show and its growth.
Afterwards i would like one grand finale where everyone come to support Ash for most important battles of his life, giving him moral strength and support of not being alone.
5 years ago no one expected that Giovanni will be seen ever again, and in BW he was revived(brought from dead) playing important role in story.
So if such minor characters can return, older ex main characters who are actually far more popular and significant to pokemon series than this examples were can surely return as well, only guaranteeing to bring success and benefit for pokemon.
Its always nice to see someone from past coming back leaving some impact on current story,interacting with new characters and getting updated. It also strengthens pokemon continuity and can potentially contribute to other character story. Like May did when helping Dawn about confidence issues,or when Gary helped Ash about Gligar problems.