Two factors above all else:
1) Give the Pokémon long-running character arcs that can be potentially as deep as or even more fleshed out than the human characters. Pokémon are the namesakes of the franchise, and making sure they have solidly constructed characters we can get attached to is more important than any human characters in the franchise. Give me Charizard learning how much his friends care and Infernape's struggles with psychological trauma rooted in a harsh past over Ash getting depressed for stupid reasons or Serena learning to find the path she wants to take any day.
And in Journeys, where Ash's Pokémon outside of Pikachu don't appear often and don't seem to have any potentially long-running character threads, that's sorely lacking. Even Sun and Moon had Lycanroc's mood swings, Pikachu's rivalry with Tapu Koko, and Litten/Torracat's rivalry with the Masked Royal's Incineroar.
2) Give Ash a consistent singular rival that pops up more frequently than other rivals that can push Ash to grow by testing his character. Ash feeling inferior to the self-righteous and cocky Gary. Being able to see himself in and learn how to grow in failure from the more mature Ritchie. Having his whole philosophy and world view tested by his polar opposite Paul. Needing to prove himself against the dismissive and power obsessed Trip. While I consider the Pokémon to be more important than the Trainers, Ash IS the lead character, and having to grow/helping his rivals to grow has always been important to the show.
Even in XY, where I argue Ash has no character arc or development to speak of, Ash plays a role in the development of Clemont, Shota, and Alan. But in Sun and Moon, he... Didn't really help anyone develop, or develop himself. And thus far, in Journeys, he hasn't done either of those things yet either. Rinto seemed like a reasonable, if cocky, human being that just needed a slap across the face. And neither Bea not Ash are really facilitating any growth in each other either.
So yeah. TL;DR, stronger Pokémon characters than humans, and good rivals. That's what I need for the Pokémon anime to be enjoyable.