Preferably one that's not a rom hack, though I dig those too. I'm looking for something a bit harder than the originals and maybe a bit more mature, but still fair and whimsical. I'd like a game with pokemon from as many gens as possible, and fakemons are fine too.
If you want something familiar but also completely different, go with Pokemon Emerald Rogue. It does what it sounds like and turns the game into a roguelike.
Pokemon uranium is usually my go-to for non-rom fan games. It's a bit more difficult than the main series games, but also packs a lot more content. Pokemon insurgence is another one that I've been trying lately. I haven't gotten very far yet, but the gyms are pretty nice in that they no longer follow a single type, but rather a theme. As for rom hacks I can not recommend Renegade Platinum enough. It preserves the base game's plot and general gameplay, but adds lots of small Creature comforts that truly enhance the overall experience (unlimited tm's, legendary pokemon are much more accessible, the npc battle Ai has been improved for added difficulty, etc.).
I know you said you’re looking for something harder and more mature, but I cant help but recommend Pokémon Bushido, and Pokémon Xenoverse. Both excellent games imo.
#1 Pokemon Vanguard is probably the one I'll play if I ever want to play a fangame again
https://reliccastle.com/threads/3066/
Fix for any lag from the developer: "The engine the game is built with has a memory leak issue I'm not currently able to fix, if the game starts to lag, save and hit f12 to soft reset and you'll be good for a bit."
Most of the Drayano hacks are pretty good. I recommend Sacred Gold/ Storm Silver or Renegade platinum. Both are reworks of their respective games with way bigger catchable dex and QOL fixes. If you want something more unique then Gaia, Glazed, and Adventure Red are quite good.
Pokemon Theta Emerald Renev is one of my favourites. It's basically Emerald, but with updated mechanics, attacks, and Pokemon, so you can have up to Gen 7 Pokemon in your team. The Pokemon list also means you can complete the Pokedex mostly on your own, and other cool features. Definitely recommend checking it out.
-Been playing radical red nowadays and really enjoying it . It doesn't add much plot wise to the original fire red but adds much more features like dynamaxing , mega evos ,galarian ,hisuian and alolan forms , removing the dependence on HMs etc . The game's fun but the difficulty can be brutal sometimes if you don't like ev training . Even I play on easy and minimal grinding mode and the game was still harder than vanilla (I am a casual player and really dislike ev training) . Still it is tonnes of fun so I will recommend it .
-You can also play pokemon clover , it is a fun game if you are into 4chan humour but at the same type it is very difficult