Main Mechanics:
    New Game/Saving
    Map scripts
    Withdrawing pkmn from daycare
    Passing down tmhm moves to offspring
    Item effects & registering an item
    EVs
    Obedience
    Level-up moves mid-battle
    Bag item usage
    In-battle item selection (requires bag GUI)
    Heal locations/black out
    World Map
    PC & Mart
    Trades
    Location name plates on enter
    Pokerus time decay
    Hidden items
    Ask if you'd like to flee a wild battle when your mon faints
    Releasing Pokmon

Trainers:
    Script commands for different battle formats
    Trainer data in JSON
    Sprite shows up for defeat
    Money reward
    Trainer sight
    Encounter music logic
    Let opponent send out first

GUIs:
    Intro/Title/Continue/NewGame
    Pokedex
    Finish Summary GUI
    Options
    Trainer card (with trainer card stars)
    Nickname GUI (and for egg hatch)
    Caught pkmn scene (requires nickname GUI)

Encounters:
    CompoundEyes
    StickyHold, SuctionCups
    Phenomena
    Fishing

Field Effects:
    Spin tiles
    Bike
    Ledges
    Blocks that start scripts (bookshelves, tvs, PC, etc)
    Countertops that forward the script (mart employees)
    Repels
    Field moves:
        Cut
        Fly (requires world map and heal locations)
        Strength
        Flash
        Dig
        Teleport (requires heal locations)
        Soft-Boiled
        Waterfall
        Rock Smash
        Headbutt
        Sweet Scent
        Milk Drink
        Dive

Data Stuff:
    Item pouches
    Dump TMHM compatibility
    Wild Pkmn with items

Sound:
    Fade BGM when in a menu
    Random cries in the overworld
    Fanfares
    Surf music logic
    Encounter music logic (and battle script commands without encounter music)
    Victory music logic
    Saving music to load next time

Nice to have (no priority):
    Changable tiles based on day/night
    Reflections on tiles
    Different day tints based on season
    Obj Following (#29)
    Attack animations with camera movement
    Pass Powers
    Move relearner & Move tutor
    Feebas/Swarm/Roamer
    Mail
    Ribbons
    Weather visuals

Sounds cool but definitely not necessary at all:
    Color lookup tables for day tints
    Move/Item effect/data integration with battle engine (not needed because you can just clone and edit the battle engine to add these things and more, like new species/forms)
    Chatot special cry (if this is done, add it as a field move)