Initial furnace placement is rarely optimal. The furnace is either crooked, or too high/low for the blocks to be placed afterwards. Then when destroying the furnace any un-burned fuel is wasted. After looking at this image below on Steam I had the idea of maybe auto flattening the ground directly below the furnace. So perhaps the ground surrounding where the cursor is pointing could level up or down so that the furnace will be level and future block building can be placed neatly underneath it.
Also, perhaps recovery of the fuel (in the form of charcoal instead of coal or wood) and some of the stone could be a possibility when destroying the furnace.
