Nothing, because I don't know how to code.
Okay, but seriously:
Ceramic tools (made of stained clay. Ever wanted an orange sword?)
I wouldn't add emerald tools/armor UNLESS, you couldn't actually use them. Instead, you give the tools to villagers, and they use them.
Underground biomes. (a few blocks under sea level the normal biome ends and the under-biome begins)
A command to put mobs on teams, as well as commands to make teams hostile, friendly, or indifferent towards other teams.
Slabs and steps for blocks that only have aesthetic purposes. (Prismarine, dark prismarine, andesite, diorite, granite, and endstone)
Make carved and chiseled stuff craft-able.
/delay <Number in ticks> <Other command>
Sets off a command after a certain number of ticks.
EDIT: And also remove bats.
