User blog comment:DuskSpiral/Developing Post V3 Fanfiction, Feedback Required/@comment-4023909-20171115123531/@comment-26397825-20171117065528

I think making your own Grimm is half the fun of writing RWBY Fanfiction; we actually have a thread (or three)  dedicated to it. Personally, I have made two unique Grimm alone (Bunker, Boomer) and collaborated with Phantomlink959 to make a few more (Nightmare and Chimera).

And since we have seen that Grimm can have elemental attacks (from the Sea Dragon and its lightning breath attack) and that merged Grimm are a thing (the Nuckelavee is the combination of a humanoid "Imp" Grimm and a horse Grimm according to the V4 commentaries), the possibilities for new types of Grimm are numerous.