It really seems to me like hoppers are underutilized, and there are a few things that could be done to make them more useful. Electric smelters was something that came to mind immediately. Have a hopper able to accept metal slag and feed it to an electric smelter. To me this makes a lot more sense than essentially hand feeding the smelter. Also make the smelter larger than it is now, more akin in size to the crematorium. Essentially what we're talking about is being able to melt iron and steel, which is between 1300C and 1500C depending on which way you're doing it. Not exactly good for people to get near. In addition to this you could add some negative event where there is a failure in the smelter and a spill of molten steel that causes a fire.