But I think you forget about the show's development in the greater picture.
Yes, from the perspective of Ash it is very repetitive and predictable. BUT look at Team Rocket. Jessie and James came as fresh recruits, and even though they "always loose", they actually conquered a lot of regions by disbanding the local criminal organisations. THEY do have an ongoing story.
The creatures and the overal background also grows. Each and every season rised the stakes. The new pokemons became ever stronger, the legends became more grandeaur. They invented The Creator. They introduced Mega Evolution what is practically making said god from any of the mons. And this is problem for the series for sake of continuity: there is nowhere to go further. You can't get higher than god.
No, i think when looking at bigger picture pokemon anime lost its identity and destination it wanted to reach.
Sure when evaluating each region individually there were made improvements in some things or new series did worse job than its predecessor.
But what good does that serve when for every step forward, story and character development goes 2 steps back.
-Such as Ash being ready to win league in Sinnoh and than undergo reset becoming less mature and experienced in next region doing worse.
-Team Rocket becoming competent villains in BW, returning back to comic relief role in next series.
-Started friendships, relationships, backgrounds and dreams about Ash friends being explored halfway through just to end up dumped and replaced with their stories never reaching culmination and resolution.
-Previously founded rules in past like only one Mewtwo existing in world being experiment of Rocket organization manipulating with pokemon genes, being rewriten messing up canon and whole history by pretending like that Mewtwo didnt existed with new one taking its place in 16th movie.
Etc.
Because in longer run what pokemon anime has accomplished really?
Thats not a good way to write for some anime.
This was why DBGT ended the series: GoKu literally became a godly power. There was nowhere to go, nothing to see. The characters threw bloody planets at each other. Heck, even galaxies.
I wander if you read my post fully than, because i addressed how in summer of this year brand new Dragon ball series focusing on Goku, Vegeta and other original characters will be made. Receiving sequel to their stories, facing never sen before enemies and unique adventure not experienced before.
There is no limit in narrative and literature in how far and how much you can do with character.
Its up to writer if he/she is skilled enough to justify further exposition of characters, story and surrounding theyre set in.
With build up acting like natural unforced follow up of what they established about someone goals, aspirations and achievements left behind before and introduce point they would try to reach with everything accumulated in past serving as springboard to reach ultimate destination.
Or not.
But in theory you can always do more with story and characters depending on someone imagination and creativity.
"Character rotation" is already present by the specials, thank you.
Otherwise problem with old companions (who are expired anyway if you ask me) is, what connection remained to them with Ash? Ok, I give you they "remained friends". So what? They don't have a shared goal anymore for longterm use. They suffice for episodic cameos, but that's it.
Like i said before as examples of Digimon Tri Adventire, new Dragon ball series, Sailor Moon remake called Crystal, Yugioh deciding to focus just on first main protagonists in next movie special shows.
Going back to franchise roots and focusing again on older characters taking them in groundbreaking new adentures is lucrative and successful strategy in increasing hype, suspense builder and popularity of long running shows like pokemon is.
Sparking interest among not only small kids, but older fans which would like to revisit their memories of childhood and follow companions they grew to like enjoying in them again or among remaining older audience keep them interested and excited to see in what way return of popular familiar faces would impact new series and nature of pokemon anime attacting older fans of some franchises in going back into them.
You ask what connects older characters with Ash aside from friendship?
In big and diverse world like Pokemon established itself to be its entirely possible for Ash and his friends life paths to collide again and experience new journey, challenges and dangers together. Since way their goals are structured allows to be inserted within current protagonist story fulfilling each other.
Whether its May and Dawn contest carer which can be advanced in parallel with Ash quest in case new region he travels through contains retired top coordinators , peculiar pokemon and contest related events which would allow them to improve their techniques, learn about more elegant and graceful ways to utlize pokemon in enchanting ways and become more successful in future contests.
Misty water master career which envelops entering water themed events like Whirl Cup, traveling and battling water experts, drawing links to E4 position and her idol Lorelei as way to rank up further . Since any tournament , powerful pokemon expert and plot which has connections to becoming better trainer or are related to water pokemon could be used as valid justification to bring her back and travel through new region if it has enough water related themes through which her dream of becoming type master could be pushed to higher level.
etc.
There could be made storyline of villain organization or alien pokemon from space coming to earth threatening to destroy whole pokemon world, towns, villages and everyone Ash and his friends care about joining him on quest to fight and stop danger which loomed itself on woirld.
Story of prof. Oak contacting some of Ash friends who proven to have strong understanding and knopwledge about pokemon to go in region Ash travels throughand help in research about ndew features like Kalos introduced mega evolution.Helping to unlock its secrets and discover what requirements need to be fulfilled to ignite this process through use of their pokemon.
Sparking their interest viewing it as great way to surpass their current limitations and make big steps in achieving their dreams depending on whether they would gain something from it or not.
Or play part in discovering with Ash about origin of pokemon life started on pookemon planet and how they came to earth helping them to better understand pokemon nature and mysteries surrounding them. Which showed to play big role in Iris case being needed to learn how to read hearts of dragon pokemon and connect with them on close level to work as onr in order to become dragon master(arguably this applies to water masters too if Whirl Cup was something to go by).
There exist plentiful of ways to make story which would conjoint Ash and his past friends careers following shared objective, mission which is in their interest to be accomplished. While advancing their own dreams comparatively through new discoveries, tournaments, new pokemon and secrets wating to be discovered.
Returning older characters to play important role in story again and receive more development is smart and desierable to do. Because they serve as bridge cionnecting new generations to previous ones, strengthen continuity and bring new fresh twists to story shaking it up and making more enjoyable to follow.
Setting up groundwork on which you can set up foundation to create long lasting impact on audience, achieve devotedness among viewers and stable ratings long term wise.
Since this way you can not only attract more of viewers toward some show through word of mouth and hype such twists make, but also have target audience get better insight in pokemon history, companion which left impact on it and better understand Ash past and experience he collected over the years. Making them as result more attached to current main characters.
You can appeal that way both to new kids and older fans having bigger sales, bigger ratings, bigger quality of story and setting pokemon anime follows etc.
Where does it says that? What he actually says "she is the master of the champion league", what simply means Cynthia is the regional Champion. In "A Trainer and Child Reunion" Ash states he'll become a master by winning a league, and becoming an Elite Four would only top his dream.
If i recall right Lucian explained to Ash and others how by winning regional league you earn right to compete in champion league comprised of battling E4 members and in case of defeating them champion itself.
Being adressed as step which would bring Ash closer toward pokemon master title. What other tasks Ash needs to achieve and conquer is unknown but becoming champion of one region wont make Ash pokemon master.
Especially when it was mentioned countless times how pokemon master means becoming strongest pokemon trainer in world and champion of oner region while may be viewed as "best" from that current region, doesnt translate in being best in world.
Just like when may rival Soledad after winning Grand Festival in Kanto became top coordinator of Kanto but not worldwide. Showing to shoot for higher positions given how she traveled to Johto to try winning GF there as well and so on.
If anything logic dictates how in order to prove yourself as most artful, knowledgeable and innovative trainer you would need to challenge an d defeat champions from other regions with best of best being left standng. Just like in real life best among best athletes and players of various sports compete vs each other to determine who deserves to carry title of world champion.
Hence why there could exist master league , Battle Frontier type of task where champions of other regions could gather and compete in one big tournament.