This may seem pretty basic compared to other people here but just wanted to give you a simple idea.
How about a robed monk/priest style of enemy? My initial idea was a hooded crow (flightless) that only wears a long robe and hood with the front exposed. There main attacks possibly being sudden dashes towards the player that you have to dodge or counter, and if you want them to be harder they could counter direct player attacks unless they do something to get around that. If they could fit in any style of area you already have planned, could have them as a cock vore enemy with a large sack, bonus is you could just close up the robe if you wanted different models for SFW.
This is just an idea I'm posting cause I loved seeing the demo you have already. I really liked packedrat and I'm very happy to see someone making a game like it. ^v^