I was thinking the same almost since I discovered the Powder Toy. Another suggestion would be centric gravity, where everything goes to the middle; or orbital gravity where everything tries to orbit the middle; or reverse gravity where everything went away from the middle. A probably more difficult one to make would be collective gravity, where objects attract to each other.
The problem with this is that that it would be hard to control, annoying but not impossible to implement, the grid system does badly with diagonal gravity (let alone all the angles for circular), and would need a good way to switch between the modes :/. I found most of this stuff out from making directional gravity in the above thread.
I could code in a solid but it would be unbelievably slow, doubly exponentially so for the size. If I can figure out a more efficient way I'll do it otherwise it's not feasible (yet )