There's a reason everyone uses FireRed and Emerald as bases for their ROM Hacks - they're the best of the best. They're still two of the most sought-after titles on the GBA to this day, and since FireRed came out on the Switch, I've been playing it non-stop again. But while they've become the go-to base for people creating new adventures, they've never been fused together to pay homage to two of the greatest Pokémon games every to grace our handhelds.
Enter ManicPacer and their ROM hack Pokémon FireEmerald, a hack that combines the two games together and makes one long story, a little like how you can play through Johto and then go straight into Kanto on Pokémon Gold. In this game, you'll play through the original story and become the champion in Kanto, before going on and tackling the challenges of Hoenn with increased difficulty.

Like Pokémon Gold, this isn't the type of game where you can just swap and change between the regions - you'll need to play through Kanto first before you get to Hoenn. Once Hoenn is open, however, you'll be able to move back and forth. All of the items, Pokémon, and everything you obtain in Kanto carries through to Hoenn too, but you'll need to re-obtain badges to be able to use the HMs you found in Kanto outside of battle. You can also access the Sevii Islands straight away after the Kanto quest too, and Cerulean Cave is accessible before the Sevii quest too.
There are updates and quality-of-life improvements like having updated battle mechanics, Pokémon following you, the option to have a night and day cycle, and wild/trainer Pokémon having raised levels once you reach Hoenn. Everything is harder, but the rewards are better. Your rival's team is much more difficult to beat, trainers' Pokémon levels are raised dramatically, but you get $1,000 more for defeating them, so at least you get something more for your troubles.
While ManicPacer has used the Pokeemerald-Expansion by Rom Hacking Hideout as a baseline to work on, this is a project that they have completed on their own and smashed out very quickly in a couple of weeks. They've said that there may be some more features like a Nuzlocke mode and extra effects heading if they have time to implement them, but for now, you can head to Hackdex to patch the hack with your legally acquired copy of Pokémon Emerald and check it out!