it has been 84 years since I played Pokémon, but the things I remember enjoying from the videogames were:
- exploring the world and solving the occasional puzzle
- filling in my pokédex (i.e: meeting new pokémon)
- reading the lore in each 'dex entry (yes, I was that kid)
... so, the battle mechanics were something I engaged with because I had to, but for me, the exploration and discovery were definitely the biggest joys. So a ttrpg that managed to incorporate some of that joy of world-discovery (for worldbuilding, may I recommend having a look at Ryuutama's collaborative town-creation sheet for inspiration? I've used it in other systems too, and it always gets players engaged in and caring about the world they're playing in), and the discovery/collection of creatures would be sweet.
It might be a lot of bookeeping, as you say, but providing not just a pre-existing beastiary, but also a way for GMs/players to create more creatures of their own to fill out the world would be neat? That would mean you wouldn't have to invent hundreds and hundreds of creatures yourself - just a good basic collection - and it would give players the means to flesh out their own settings and make them more personal.
Some players might be after the battle mechanics modelled on Pokémon's own, but I would personally find that a bit crunchy in ttrpg format. It works in videogames, but it's a LOT to keep track of at the table. Like, I even played Pokémon White with an actual printed out chart of type-advantages because I couldn't keep all of that stuff in my head.
Potential principles:
play to discover new pokemon
play to discover new information about pokemon
play to discover the secret history of pokemon
This seems in line with the anime and movie especially. I should have said this instead of something about choosing which pokemon attack each other. I think there are a lot of people who engage with the concept the same way.