Maybe Ash finally takes up that offer on becoming a Frontier Brain, succeeding Brandon as the Pyramid King or being the roaming Brain in another region (and a way to use the Sinnoh/Johto Brains besides Palmer should future instalments revisit him). It'd fit his interest in travelling and be more than just becoming a gym leader plus be a nice callback to how the Alola games have you defend your title by fighting randomly decided NPCs.I don't know if I could see Ash managing his own Gym in large part because he clearly loves to travel. Managing a Gym wouldn't prevent him from traveling whenever he wants to, but it would be a bit of a limitation since he'd have to stick to primarily one location. Plus, after they made Paul into a Gym Leader, it seems less likely for them to do the same thing with Ash. I still think having Ash continue to travel off-screen would make the most sense. It would fit with his character and leave the door open for him to appear down the road whether it's in the new series, another mini-series, a movie or whatever.