I've been keeping an eye on this for quite some time. Turns out the guy actually delivered, it's probably the most impressive hack I've seen so far
Quick rundown

Multiplayer features:
>can play Emerald with friends (up to 4 people)
>all the players will roam the same world
>can trade with them from anywhere
>can fight every NPC in multiplayer mode (including gym leaders), they'll turn into multi-battles
>...which means the whole story can be played in splitscreen with your bros
>PvP is enhanced (can fight from anywhere with modern mechanics, all known modes are included)

Updated stuff:
>every Pokémon is included, Pokedex has 900+ mons (even Alola and Galar forms are there)
>you can actually catch 'em all
>no, it's not just a pile of shallow reskins, they're actually fully implemented
>battle system updated with modern mechanics (up to 8th gen)
>mega evolutions and Z moves included
>all items are also updated (up to 8th gen too)

New stuff:
>can choose between 10 avatars from all generations (can also switch trainer whenever you want)
>can choose between 40+ starters
>up to 6 pokémon can follow you around (HG/SS style)
>you can actually see and approach wild pokemon roaming in the grass/caves/sea (all of them have dedicated overworld sprites)

Quality of life improvements:
>can rematch almost all trainers whenever you want
>new respawn system for items and berries
>no need to waste space for HMs to progress (just get the item and you'll be able to use the move whenever you'll need it)
>nursery can be accessed from the PC, number of pokémon simultaneously allowed there is increased (there's a new menu to check how they're doing)
>no need to bring your eggs around to hatch them, you can put them in a special box instead (up to 30 eggs)

New options:
>multiple game modes (normal, randomized, cheat enabled)
>multiple difficulty levels (five, from very easy to very hard)
>can set shiny % rates before starting the game