Things:

(List these like Fundy listed the changes he made in impossible difficulty)

Defo crystals under lava level
Defo tools that are more powerful than diamond
Defo ring that is cool
Stalagmite (anybody in the comments who says its a cobblestone wall will be smited.)
More annoying rocks like diorite and granite
Old miner camps
Classic rubys (will be talking about it later)
Three types of asteroids
Ruby alter so you can turn rubys into XP
Throwable lanterns which you can throw (obviously(the throwable lantern is satisying is HECK!!!))

Thank you to everyone who gave ideas.

(There was going to be a miner zombie, but the texture got corrupted)