I was wondering if you'd be able to assign 1.12.2 items the rarity that they have in 1.13+. While I'm not gonna expect this for a while, I do hope it'll make it into Future MC.

Here's a list of 1.12.2 blocks and items with their 1.13+ rarities for your convenience:

Uncommon (Yellow):
[
Bottle o' Enchanting
Dragon's Breath
Elytra
Player/Mob Heads
Nether Star
Totem of Undying
]

Rare (Cyan):
[
Beacon
End Crystal
]

Epic (Magenta):
[
Command Blocks
Dragon Egg
Structure Block
Structure Void
Spawner
Barrier
Minecart with Command Block
]