I think the best way to hint that Sandman is still "out there: is the following. He is defeated by getting blown up high in the sky. At the end of the movie after the final swing the camera shows that all over the city, when you look really, really close up to the top of a building or the pavement of a sidewalk, or an alley, you see a very small amount of little grains of sand that look like they are all blowing in a direction pointing to the same spot somewhere. It's to hint that he's reforming, or rebuilding. Pulling himself back together. When he does come back later on in the series and if he comes back, it could be explained that it took himself years to put himself back together. He's angry, crazy, and furious to having spent years like that in his own prison sort of. Unable to talk, or do anything but blow in the wind.