The one thing that all these 'Adult-Oriented Pokémon' fan game ideas lack, and which kills them in the end, is that they never seem willing to break away from... well, the Pokémon. If you want a fan game that is less childish and takes on a demeanour that is more appealing for adults and all their differences compared to kids, you gotta drop the canon Pokémon style. In fact, you gotta edit the original designs to conform (more or less) with reality. You gotta scrap ~80% of the series' foundational concepts, mechanics, and lore. You gotta reign in the 'grand adventure' schtick of every main-like story and put the player in something more mundane. You must, in the end, make Pokémon not so.
As an example, how would a 'redesign' of Kanto look and act like in this spirit? One thing I think would aid in grounding Pokémon in a more relatable manner would be to acknowledge that most Pokémon regions are based off of real life regions, so in the case of Kanto... just have the entire game take place in a fuck-huge Saffron City. Or, if you don't want localized names, Yamabuki-to (棣棠都). Either way, it's the capital of this PokéJapan and you, the player, start the game awoken from your slumber in a hotel room at either not-Narita Airport or not-Haneda Airport (game counterpart dependent). Have the player take the role of a traveler, or a guest worker, or a returning citizen if that is your thing.
Fantasy is all fine and dandy, but we don't need too much of it here. The technology associated with 'Pokéballs' is kind of absurd, as is a ton of other stuff in the main-like games, so why not make do with a three Pokémon limit? And have it be size-dependent too, since we're getting rid of Pokéballs completely. Your team will walk with you out in the open, so Pokémon that are too large will be disallowed entry into certain places and take up the other 'slots' in your team. Owning a Pokémon is a privilege or adulthood, proper training, and licencing.