Placeholders

• World Time
• World Difficulty ( returns difficulty as string )
• Active Hotbar Slot
• Slot Item ( returns the current item in the given slot as registry key (resource location) )
• Current Player Health
• Max Player Health
• Player Health In Percent ( returns value between 0.0 and 1.0 (0% - 100%) )
• Current Player Absorption Health
• Max Player Absorption Health
• Player Absorption Health In Percent ( returns value between 0.0 and 1.0 (0% - 100%) )
• Current Player Hunger
• Max Player Hunger
• Player Hunger in Percent ( returns value between 0.0 and 1.0 (0% - 100%) )
• Current Player Armor
• Max Player Armor
• Player Armor In Percent ( returns value between 0.0 and 1.0 (0% - 100%) )
• Current Player Experience Level Progress ( returns value between 0.0 and 1.0 (0% - 100%) )
• Current Player Experience ( returns total current experience )
• Current Player Level ( returns current level, not experience )
• Current Mount Health
• Max Mount Health
• Mount Health In Percent ( returns value between 0.0 and 1.0 (0% - 100%) )
• Mount Jump Meter In Percent ( returns value between 0.0 and 1.0 (0% - 100%) )
• Boss Health In Percent ( takes index of boss bar ; returns value between 0.0 and 1.0 (0% - 100%) )
• Active Bosses Count ( returns current boss bar count )
• Active Effects Count
• Active Effect ( takes the "index" of the active effect ; returns the effect's registry key or an empty string if no effect is active for that index )
• Current Title ( takes title line (1 or 2) ; returns empty string if no title is shown )
• Current Player X Coordinate
• Current Player Y Coordinate
• Current Player Z Coordinate
• Current Server IP
• Player Attack Strength Percentage ( value between 0.0 and 1.0 (0% to 100%) )
• Gamemode ( returns game mode as string )
• Player View Direction ( north, east, south, west )

Requirements

• Is Hotbar Slot Active ( takes slot as number )
• Is Slot Filled ( takes slot as number )
• Is Left-Handed
• Is Clear Weather
• Is Raining
• Is Thundering
• Is Snowing (only if possible in an easy way)
• Is Player Running
• Is Player Sneaking
• Is Player Swimming
• Is Player Jumping
• Is Player In Biome (takes registry key (resource location) of biome)
• Is Player Under Water
• Is Player In Water
• Is Player In Lava
• Is Player In Fluid (all types of fluids, also modded ones)
• Is Player In Dimension (takes registry key (resource location) of dimension)
• Is Entity Nearby ( takes radius in blocks and registry key of entity (resource location) )
• Is Player Riding Entity
• Is Player Riding Jumpable Entity
• Is Player Riding Entity With Health
• Is Gamemode ( takes gamemode )
• Is Effect Active ( takes registry key of effect type if possible )
• Is Any Effect Active ( returns true if at least one effect is currently active )
• Is Player In Powder Snow
• Was Player In Powder Snow
• Is Player Wearing Pumpkin
• Should Show Attack Indicator ( if player attack strength is smaller than 100% )
• Is Player Flying With Elytra
• Is Player Flying (Creative or Spectator)
• Has Player Absorption Health
• Is Player Withered
• Is Player Fully Frozen
• Is Player Poisoned
• Is Difficulty ( Takes difficulty as value )

has caps locked or not
has block num or not
system volume
game volume (pid volume)

player is flying
player is death
player direction
player raycast target (what is looking)

(1.20.5+) current server stored cookies
current server ping
current server tps

I'm not asking for anything here but... these placeholders would work really well for some kind of HUD overlay or replacement.