There are quite a few characters about who i would like to see specials/spin off made. Having potential for further development and plausible direction through which sequel of their lives could be made.
For example i would like to see follow up on Cilan adventures in Hoenn. Cilan dream of becoming top connoisseur was very little focused on which is shame because i thought it was unique and interesting kind of profession connected to pokemon. Maybe we could see Buirgundy too and get proper closure between her and Cilan facing of each other in final confrontation. Cilan had lot of unexplored things about character and unfinished story deserving to get more exposure imo.
Brock getting some follow up on his new doctor career would be welcomed too, see him bond more with his large family and apply his accumulated knowledge on pokemon. Or go through stage fright before entering important test, confront evaluators to show in practice his knowledge etc.
I always felt he was robbed of from proper focus in DP special where Forest got more attention.
There is also May. She never got follow up after departure which is shame, because coverage on her Johto adventures had material to make something memorable from it. They could show how her contest journey there concluded, relieve some of amusing and sweet dynamic with rivals like Drew or always vengeful Harley etc.
Or introduce to us new adversaries she met on travels, building on her contest style and learning about new ways how to improve her strategy. Perhaps we could also find out where she went next after GF in Johto finished.
Speaking of new pokemon i imagine Lilligant or Mienshao would be interesting choice for contests.
Misty is character i would definitely like to see again having plenty of untapped potential to do more with water master career, her pokemon, past and family having untied things about her master career and in what direction she will go. Her flamboyant, sometime flirty and outspoken personality always added certain charm and spice in interactions and i would be interested in finding out what has been happening with her for all this time.
Such as how much she improved as trainer, what new pokemon she acquired, and if she came closer toward her dreams. Seeing her doing solo journeys to catch new pokemon or develop battle style overcoming water weakness to other types, learning from other water specialists like Wallace or Lorelei training with them on Seafom islands for example gives material to make interesting arc from it..
We could see them preparing special tasks(battle competitions or races) and evaluators testing her in learning to extract full potential of her pokemon on surface, to evaluate if she has potential to be E4 of water pokemon one day recommending it to league committee based on performance. We could also see her enter water competitions like Whirl Cup advancing water master goal winning them, even meet Dawn when going to Johto again since she went there too allowing to have them meet each other. When such thing wasn't meant to happen in main series.
And deal with her own unsolved issues like judging pokemon mainly by looks(i can already see small pokemon like Surskit as half bug likely causing conflict in character struggling in accepting it despite being partially water adding for great character development making further steps forward). Or having trouble in expressing feelings and what bothers her keeing issues to herself even when on her harm(as showed with sisters in not letting them know what she finds frustrating and irritating about their behavior and running from obligations as evidenced when speaking with Max about it).
There definitely exist multiple ideas to take her forward, with purpose of building up on her story and dreams of becoming top trainer with story being unfinished in many areas(based on abrupt departure, plots left hanging in air and undefined future).
Specials about Iris origins would be appreciated too, to get understanding about her childhood and if she even has any close family left. Or to see where life would take her after leaving Ash. Who knows maybe her Axew would evolve in spin of series too.
There is also another option. Like coverage on Gary and Paul trainer, researcher careers. This is starting to border with headcannon, but i always wanted to see those two meet, interact and measure their strength by going on each other in full battle. Afterall they are some of toughest trainers Ash faced in his life giving him very hard time.
