Pokémon

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 Infinite Fusion is all the rage right now, the gimmick is kind of cool, and you can choose various difficulty levels.

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.

my recommendation is Pokémon Unbound. really enjoyed my experience with that one!

#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."

#2 Pokemon Eon Guardians is good except it has issues with lag https://reliccastle.com/threads/5445/ This is supposedly the fix for the lag

#3 Another that I know of is Floral Tempus https://www.rebornevo.com/forums/topic/38862-pok%C3%A9mon-floral-tempus-ex-v194-released-12-gyms-elite-4champion/

Pokemon Eternity https://discord.gg/TZWNXrxK

More mature, traditional mmorpg style questing, real time battles against NPC's, wild pokemon, and other trainers.

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.

I had a lot of fun with Pokemon Xenoverse. I think it ticks a lot of your boxes.

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