If I'm missing anything notable, please let me know.

Ghoulslash: overworld expansion (fixes by Deokishisu), costume system, auto-running (adjustmenst by Jaizu), repeated medicine use, dynamic Marts by Badge count, bag sorting framework, options menu scroll arrows
DizzyEggg: colored stats by Nature on Pokémon summary screen, faster text rendering (fixes by AsparagusEduardo), options menu scrolling, Exp. gain from catching Pokémon
TheXaman: registered items list, options menu expansion framework, self-trading (fixed names by Zeturic), text font option framework, Match Call option
AsparagusEduardo: units system option (with Lunos), modern white out money loss calculation (with tweaks by lightgod87)
Citrus Bolt: legality/personality seeding code, Nature/gender checks for chain encounters, saving data to Trainer Hill save sector, e-Reader card verification/offset info
ExpoSeed: reuse Repel OW prompt, dynamic overworld palettes (public branch; original code by Slawter, algorithm by CompuMax)
huderlem: triple layer metatiles (with fixes from SBird and Citrus Bolt)
Lunos: one-time TM purchases
Blurose: modern Exp. system
Zeturic: Pokémon summary screen wrapping
Jaizu: automatic lowercase for text inputting
cbt: simultaneous multi-directional inputs
Ketsuban: bike switching
AkimotoBubble: framework for displaying IVs
Tustin2121: framework for full save block compatibility

Beta testers: Hobusu, Akiak, Cael Thunderwing

All of the people who've contributed to pret's pokeemerald decompilation.
All of the people who contributed to the main decapitalization branch (Wyrserth, Lunos, and AsparagusEduardo, with tweaks from me).

Also, thanks to the above and countless others who've made and edited tutorials to both implement those features and gain a better grasp on the decomp environment in general.

Tools used:

VSCode
Porymap
paint.net
Irfanview
Audacity
Tilemap Studio
Sekaiju