I wasn't planning on adding a lot of new creatures, but I have ideas for two:

1. Displacer Beast -- does not render a model, but has a EntityDisplacerBeastImage entity that wanders within a couple blocks as does. (So it appears to be 0-3 blocks from where it is).

2. Tarrasque -- a full boss level creature, probably merging some D&D with the traditional creature of French legend. Problem -- its main powers are resistances, making it a melee super-tank, rather than having a special attack power. Probably with a diamond shell (drops a bunch of diamonds). Not technically unique, but only one allowed in a world at once.

Compare:
https://en.wikipedia.org/wiki/Tarrasque_(Dungeons_%26_Dragons)
https://en.wikipedia.org/wiki/Tarasque

A couple other ideas:

3. Ghost or spectre, something to attach the D&D trope of "only harmed by (magic or) magic weapons" to.

4. I have some really cool ideas for maralith / type V demon. Six rapid hits in melee range; archvile like attack at range.

EDIT: I have added sound for all these creature to the repo some time ago (they are not referenced in code). I don't know if I'll ever make them, but if I come back to this later (and some version hasn't been added yet) its still not impossible.

The big problem I have is that I don't think I can hand code models in the same style, and can't get any model making software to work. So, I'm kind of stuck on adding any kind mob.

@Sunconure11 Feel free to mention your own creature ideas here -- though I will probably work on my own and leave you to add any you think would fit.

I've got some, but I will figure out how to code new mobs first.