I think you should be able to right click with a slime ball in your hand to place it on a surface of a block. This would act as glue to stick blocks together. This way you could suspend gravel from the ceiling/wall and form contiguous series of blocks (such as platforms) that can be pushed by pistons. When a piston pushes one block that is stuck to another, then the other block moves with it.
I think this would be a simple yet useful addition to the game.
Tell me what you think!