A list of several ideas.

I will put these here as a list:

- Ability to add custom music to dimensions.

- Ability to add a custom sky texture or the sun and moon for dimensions.

- Ability to play custom music for mobs while they are alive (Starting as the mob spawns and ending when the mob dies, and also repeating if the song ended but the mob is still alive).

- More advanced Mob AI tasks.

- Being able to add custom tool types that have custom effects when swung, have a custom swing speed, and a custom damage.

- Being able to put buckets of custom fluid in a crafting recipe.

- Custom Music Discs.

- The ability to run commands with procedures, but not show the result in the chat, and not need to have cheats on.

I might add more if i think of some.