New mob - Devil Sheep, aggressive, larger than regular sheep, slow, but does high damage, also inflicts fire damage.

Maybe some new plants as well

New ores that can only be mined with high-tier pickaxes

Progression in game will look like this: Overworld → Nether → End → Spooky Sands → Frozen Wasteland → Nether (again) → whatever is next

And a new boss