In all honesty if you are looking for something new but with a classic feeling try Pokemon Gaia. Mostly it's probably the most recommended GBA rom hack at this time. The region has a super polished feeling while the story pace is great and updated movesets make it feel like a modern game on old hardware. Also Fossil story line goes crazy good.
Bro if you haven't played Pokemon Radical Red then start there. It's damn hard though. Like, "rage quit at 3 AM" H.M. hard hard. But it's super fun if you're into the mechanics of competition. Updated dex, new abilities, mega evolutions basically everything a normal GBA game wishes it had.
I swear everyone keeps recommending Radical Red like it's the only hack ever made . . . . .
Try Pokemon Unbound instead. WAY better story, smoother gameplay and you can actually enjoy oneself instead of losing 27 times to a random trainer on Route 2.
Plus the graphics and QoL stuff is insane for a GBA hack.
Not gonna lie Unbound is overrated IMO The "open world" thing makes a mess.
If you are looking for something clean and Adventure-ready then Vega (with English Patch) is super cool. The fakemon designs are weird at first but they grow on you.
And b4 someone says "it's to hard bro" that's the point. It's a challenge that it's supposed to be.
I'm just here to say:
Emerald Rogue Is The Crack Version Of Pokemon.
Every run is a new one, you basically get a roguelike Pokemon adventure game and it makes me somehow game it for 5 hours without being able to realise i just did not eat.
If you are looking for some super replayability, this is THE one.