It isn't easy to imagine Pokémon ending, especially for how much momentum the franchise has. In that same vein, it's difficult to imagine a scenario where Ash stops traveling to new regions and working towards his goal of becoming a Pokémon Master. However, all good things must come to an end sooner or later.
There may very well come a day when Ash reaches the end of his Pokémon journey. At the very least, he'd be done traveling to new regions to compete in their Leagues. Whenever that happens, and he has to grow up, he might have to get an actual job that allows him to support himself. As for what exactly Ash Ketchum would do for a job, that's anybody's guess.
It's possible that he'd hoped to become rich and famous by becoming a Pokémon Master. If that's the case, that would seem to suggest becoming a Champion for his professional job. However, from what's been shown, becoming a Champion is more of a title than any sort of paying job; after all, Ash is already the Champion of Alola, yet he's received no money to speak of for it. If Ash wants to earn a guaranteed, sustainable income, he'll probably have to look somewhere else.
If battling is really a requirement, a few options have been made available to him. The first thing that might come to mind is becoming a Frontier Brain. Ash had been offered the position once before, but he turned it down so that he could continue his travels. He was told that if he ever changed his mind, the position would be open. If Ash's only reason for not becoming a Frontier Brain was his travels, then taking the job after he's had his fun would be a logical next step.
Another possibility would be for Ash to become a Gym Leader. He could establish a Gym right in Pallet Town. He'd be responsible for helping some of the new Trainers fresh out of Professor Oak's lab get their start on Kanto's Gym Challenge.
However, Ash might not be content with staying in one place for too long. If the anime has taught audiences anything about Ash, it's that he prefers the nomadic lifestyle of a world traveler. To maintain this lifestyle, he could become something like an International Police officer like Looker or a Pokémon Ranger like Solana. Even if he stops training Pokémon, he'd probably want a job that lets him go to different places.
There's also a prevailing theory that Ash could someday become a Pokémon professor. Between all his first-hand experiences with Pokémon, including Legendary and Mythical Pokémon, he probably has more practical knowledge of the Pokémon world than most seasoned professors. There's also his strong connection with Professor Oak, among others. If Ash became a professor, he could continue to travel and interact with Pokémon as part of his research.
In Truth, the question of what Ash would do after becoming a Pokémon Master has been asked before in-universe. In Pokémon Black & White Episode 52, "Ash Versus the Champion!," Alder asks him about his plans, and Ash admits to never having thought of them. At the end of the episode, Alder recommends that Ash and his friends aim for coexistence among people and Pokémon, and therein might just be the best answer. As long as Ash can continue working towards this goal, he'll probably be happy.