Game Tech journal!
I was playing Gmod at my friend's place, and while he was showing me some of the SENTs (Scripted Entities) he had downloaded, I noticed that one package had similated liquids.
Using HL2 models, the SENT pack had small and large versions of soda and beer bottles, plus a water jug and milk carton. You use the containers, and if they're at a certain angle, a particle effect 'pours' out of the end of the container, in colours to match the liquid it simulates. You can also use it again to close the container, stopping any remaining liquid from coming out. I think it also works on an angle basis, so if the level goes down enough, certain angles will stop having liquid pour. I think this is great. A little refinement, like a denser particle effect, a pool on the surface below, and a pool fade effect that changes how long it takes depending on the material it comes onto, and the light level from the maps light_enviroment that hits the area the pool is in, and you have a respectable liquid particle effect.
Add to that a brush entity that fills its volume with the chosen particle and then removes itself on map start, a prop_container with differing model and liquid choices, the ability to see the particles inside transparent props, and a new console command to update the particles in hammer form the game (think the command to update prop_physics), and there you go.
Just make sure you recode the prop_container to make +use open/close the container and switch submodels appropriatley, and that's the peak of innovation.
Are you reading me, Valve? Got it? Good. I expect to see it either in Episode 3 or HL3.