This is something I’m sure a lot of people can only dream of doing: full-scale water simulations with live particles, if possible to be simulated by the GPU. These are not just GPU sprites that can bounce off surfaces, but particles that can fill a container with water, dump it out, and interact with environment physics to create interactive water-based gameplay. This requires the particles to actually retain volume and roll down surfaces (or G-buffer normals) instead of just bouncing off.
I remembered UDK had basic implementation of Apex particles for fluids, but that was a very experimental, buggy, extraneous thing that didn’t really work all too well. Some more guts behind the smoke and mirrors would be lovely.