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.

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

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

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.

Pokemon Eternity https://discord.gg/TZWNXrxK

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

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

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.