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

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.

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

Pokemon Eternity https://discord.gg/TZWNXrxK

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

-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

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.

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.

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