Could release beta exe + editor where maps are shared on the web, looking for emergent gameplay.
(bit a la minecraft beta)
About optimisations ands ideas:
- Did you check fluids related paper (lots of paper on optimising those)
Or even that nice latest snow thing http://www.youtube.com/watch?v=9H1gRQ6S7gg paper here http://www.math.ucla.edu/~jteran/
- Voxeks route, at it's proven it does work real time, see http://www.volumesoffun.com/introducing-cubiquity/ for instance.
- Did you tried using gpu for computation. (as your apps doesn't look too much demanding on graphics...)