Probably uneventfully by this point. In all likelihood they'll have Ash win something and such, and then possibly be retired, alternatively it'll just be shelved. Perhaps after a strange mix-up Pikachu becomes a trainer and wins the League, in the process showing every other trainer around there how it's done, and then it does a Mewtwo and invites strong trainers over to face it. Ash is accompanied by Brock and Serena, and then a Black Hole is formed because those two are basically the same person, leading to an end to the anime along the lines of Space Runaway Ideon. Maybe Ash is eventually disgruntled about all of the Leagues, and as a result they resolve to become Kalos Queen, and as that's seemingly much easier, they do that and then the anime ends for shame. It seems they're more likely trying to figure out how to keep the anime going for one or two more seasons once they have to do that, though.