Perhaps motion blur not linked to camera movement would be interesting, if at all possible.
During my holidays I got this great idea (and no computer to test it) to make an avalanche (and blur grass/trees, etc) by making 2 frames where the second frame had a different set of values (smaller/less stones, or an eroded version of the same basic terrain). Rendering the first with motion blur checked would (so I thought) calculate a motion between the two frames' values, and render this.
Nope, didn't work. If it would, there would be masses of things you could use it for, like the soft transparent water in long exposure photographs, wind blurred fields of soft grass, pieces of ice falling of glaciers..................