Fully dynamic, particle based fluids are something I'd like to put in, but they are pretty far out there chronologically speaking. They would have to wait until after I get some more fundamental functionality in and performance higher (multithreading and so on). I have tinkered with the idea of doing some GPU-side simulation, though its not my favorite development environment. It probably would be the best for this kind of mass-particle system though(http://www.youtube.com/watch?v=RqduA7myZok), but I'd like to focus on the core physics for now. Something closer (though still without any hard deadline) would be heightmap-based fluids with interactive propagating waves.
Might have to wait a while for physically simulated rain
Hehehe was just curious if you had thought about it. Very cool demos there on youtube, would be amazing to have that flowing down a street in real time