When I go to crop things in Shotcut, I can only crop them in either squares, rectangles or circles. I think it would be a very good and helpful feature if Shotcut could allow crops to be more like a lasso tool where you crop edges like making a right triangle crop.
You can use Mask: Draw (Glaxnimate) and Mask: Chroma Key to do other arbitrary shapes. You might need to use Alpha Channel: Adjust with Invert checked with some of these.