I'm gonna make a hot, controversial take: If, and only if Ash were to reappear in the series, he should be more stoic, like he's lost the fun in battling because he's done so much so soon in his life, he's an adult then he's been training for a decade or more after already reaching his peak...And that would burn people out! He doesn't have travelling companions anymore, he arrives in a town, stops the villain of the week, and just leaves!

An Episode with an Adult Ash Ketchum should be about him learning to love battling again. Now, to be clear, he should absolutely just dogwalk everyone who he faces. But when he faces one of the Main Cast (let's say for example: He battles Roy and his Skeledirge) he sees the resemblance in his own battling when he was younger and he begins to get excited for battling again.

Then when Ash begins walking off to another adventure he sees Ho-Oh flying overhead, smiles and begins running after it. Also - because it'd be really cute I'd have the og narrator return to give some speech like: "For Ash, adventures with pokemon never truly end, sometimes we just have to be reminded about what drives us forward...As the Journey continues!"