Seeing as it says that theres going to be a hell realm, I'm guessing it's probably doing to be demonic mobs.
Maybe some sort of really fast (REALLY. FAST.) demon? Or fire zombie, like people have wanted for a while.
GOLEMS! We need golems that use different elements from the game! Like dirt golems near the surface, rock and steel more underground, and fire near lava! So the deeper you go, the stronger the element!