Pokemon has always been and will always be for children. That's not to say that adults can't enjoy the series, it's just that kids will always be Nintendo's main demographic.
The stories always feature a happy ending, and never stray anywhere near "adult" themes like loss or death. Even after being beaten, players simply faint and reappear at the nearest Pokemon Center.
So ... what would happen if someone made a Pokemon game that was targeted toward adults?
A modder by the name of Nathan Gunzenhauser is trying to answer that question with Pokemon Apex. It's not that the game focuses on mature content like sex or gore, but more adult-oriented themes: traditional Pokemon games typically center around something like "friendship beats everything" — in Pokemon Apex, however, Gunzenhauser is looking at themes like death and abandonment instead.
That may sound like a huge departure for the series, but that's only the beginning: everything, from player progression, to Pokemon selection to general battle strategies, have all been changed. As Gunzenhauser puts it:
"The game is intended to be somewhat difficult, meaning that you will need to think about your strategies and make conscientious choices about your team in order to progress. Playing through the game haphazardly will get you into trouble."
He's serious, too: combat has been completely reworked, with a huge focus on actual strategy. It's a lot more complicated than it used to be, but Pokemon has always been built with the rock-paper-scissors Type Advantage system. It certainly works well, even after 20 years, but it's also easy to take advantage of. Pokemon Apex is attempting to fix this by not only raising the stats of all Pokemon, but by tweaking their movesets to take more advantage of their unique characteristics.
It's not just the combat that's been changed, either. Player progression in Pokemon games is typically limited by how many gym leaders you've beaten — in Pokemon Apex, however, players need to unlock new areas and complete sidequests before moving through the game. If anything, it seems as if Apex will focus on a more traditional RPG structure than the original Pokemon games do — which, at this point, could end up being a welcome change of pace.
As for the "mature themes," don't worry — you won't be watching your Pokemon beat each other to death. Aside from a few curse words, the mature aspect of Pokemon Apex is in its story: you play a high school student who's mysteriously transported to a world full of demonic monsters and forced to battle a cult over the fate of both your home and this new universe. Again, it's far more in-depth than any of Nintendo's Pokemon stories, and is exactly the sort of thing for which fans have been asking.
Sadly, there's a good chance that Pokemon Apex won't make it much further in development. Nintendo is notorious for cutting down fan projects like this, and with its family-friendly image to consider, there's a good chance that a Pokemon game with swearing and demons in it will face a similar fate. It's a shame, too: the idea of a Pokemon game that's aimed at more advanced players is a novel one, and something that Nintendo should at least consider exploring.
For the time being, you can still download Pokemon Apex for free over at Gunzenhauser's devblog.