Features

Setting Expectations

Heart & Soul was designed as a traditional Pokémon experience, akin to the main-series games. It is not a difficulty hack. The challenge settings are a bonus, but the game is not balanced around them and will not be in the future.
Heart & Soul stands on the shoulders of giants. I am not an artist, so much of the art and some of the code for this project was drawn from open source assets. I did my best to credit all sources below and in the game credits, but if you find any that I missed, please let me know in the Heart & Soul Discord.
Heart & Soul has been rigorously playtested, (shoutout to the playtesters) but there still may be some small bugs. If you find any, please report them to the Heart & Soul Discord.
Heart & Soul is not intended to be 1:1 with GSC or HGSS. A lot of it is. But some differences are intentional.

Generation 1–3 Pokémon, plus their later-gen evolutions (excluding the Regis)
Full Johto story and Kanto postgame from HGSS, including the Kimono Girls and Eusine
Following Pokémon
Physical/Special split is toggleable
Overworld background Pokémon allow you to easily see the notable encounters on each route
Overworld background Pokémon in cities, towns, or areas with no encounters are just for the vibes
Day/Night system with variable encounters
Dynamic overworld palettes
HGSS music
Battle Frontier, Trainer Hill, and Trainer Tents from modern Emerald
Highly varied trainer teams and encounters, based on Crystal Legacy
Pokémon that can learn an HM do not need to have learned it to use the HM field move
Timekeeping does not track days of the week; everything is progression based
16 Gym leader rematches
Two Safari Zones
Unique surfing Pokémon sprites
Customizable shiny rate
Fairy type is toggleable
AutoRun and FastSurf are toggleable
Quick run from wild battles using button combination
Ball prompts for quick catching

2.0 Features

Rebuilt the game and all features on pokeemerald-expansion. This means saves from before 2.0 are not compatible with versions after 2.0.
Restored Surf and Bike music, with the option to disable
Abilities up to Gen 9, including Hidden Abilities
Items from up to Gen 9 available, including Shiny Charm (not all items)
Randomizer has all mons Gen 1–9
Hidden Items + Itemfinder
Overhauled Kanto maps and tilesets
Kanto rematch trainers
Kanto in-game trades
All PokéGear rematches from GSC (and some new ones), including gift items, with the option to disable
Full PokéGear Radio with all channels from Crystal (and some new ones)
PokéGear Condition menu
All 92 HGSS TMs
New postgame content
Kurt's Berry Balls boost catch rate by custom-defined type affinity if more impactful than the vanilla boost
GB Player Key Item to toggle Gen 2 music and sounds
Removed all bike reaction time puzzles
Added ice Path boulder puzzle
Mom's Savings System
Voltorb Flip
Entei, Raikou, and the new Kanto roamer are all independent
Choose one Lati to roam Kanto. The other will be at Southern Island.
Roamers no longer intentionally avoid the player's route
Added a sound cue and animation when the player enters the same route as a roamer
Added a clock to the start menu
Added Optional Bonus Content: TELEPORTER, accessible from the Kanto postgame
Room Decoration (dolls only) in New Bark Town
Spiky-eared Pichu event in Viridian
Added Optional Bonus Content: ISLES, accessible from the Kanto postgame
Slowpokes grow their tails back when you beat the league
Tutors and trades can utilize your PC Pokémon
Added Optional Bonus Content: SINJOH, accessible from the Kanto postgame
Can fly to more routes and Pokémon Centers
Rocket takeover only happens after the 7th badge, no matter where you get it
Buffed the Kimono Girls
Various UI/UX improvements, including Shiny Markers for colorblindness accessibility and shiny-hunting QOL
New Battle Backgrounds: Cave ponds, snowy routes, snowy mountain, snowy cave, volcano cave, new beach

Differences from GSC / HGSS

Level curve is greatly improved
Apricons are replaced with berries, including to be made into unique Pokéballs by Kurt
Swarms are replaced with a 5% Hoenn encounter on each route in Kanto, which can be boosted via the Hoenn Sound radio channel
Ruins of Alph back room requirements are slightly altered
No Pokéathlon dome
Modern Emerald Battle Frontier and Battle Tents
Unlike vanilla Emerald, "Level 50" will set any Pokémon level 1–100 to level 50, and "Open Level" will accept any Pokémon as-is