I'd love to see an enemy or boss type that's a bipedal, swole AF goat warrior thing similar to the demons featured on many bestial war metal album covers. I can't post an image link for some reason, but Google is your friend. His primary attack would be this massive upside-down cross that he'd beat you with. He could also impale you with his horns or trample you with his hooves.
Or how about another enemy type: an order of monk-like demon hunters whose zealotry is so intense they commit atrocities against the local human population? I imagine these guys would be similar to the Oprichniki, which was Russia's secret police during the reign of Ivan the Terrible. Read up on those guys if you're in the mood for some legit historical grimdark. They're like something out of Warhammer 40,000.
This may be outta left field, but another brutal enemy type I came up with was a primordial, caveman-like devolved mob who've been overtaken by Mundus or whoever's power, but not completely transformed into demons yet. They'd look and operate similar to the Forsworn from Skyrim, as cliche as it sounds, or Tyler Durden's wet dream.