NiSi Optics, known for their filters and filter holders will be launching an RF mount wide angle lens soon.
All I have received about the lens is a picture of it, and I can't break down what exactly it is by the image alone. It looks like it's an f/4, but the focal length is a bit of a mystery. It's probably safe to assume that it'll be a manual focus lens
If it's priced right, it should do well.