Perhaps a lens that goes to f/32? One of the goals of waterfall shots is often to maximize your exposure length. When shooting recently I got surprised by the Sigma 35mm Art only going to f/16, instead of the more common f/22. I think there are some lenses that do go to f/32, but I'm not sure how much the diffraction affects the sharpness and how you judge that vs. the extra long exposure blur. Since you're stopping down so much, you don't need to start with the best lens... except of course if you're going for the TS-Es.

So yeah, a lens that can stop down extra far that takes one of the filters you have.