Basically a combination of RTG furnace and Blast furnace: Pellet refill and waste ejection, and ingredient slot side I/O configuration. And it would be nicer if shift-clicking could shove pellets into their slots instead of ingredients slot.
Also I found that pellets stop to decay if the furnace is filled with a recipe that has no outcome, but decays normally if empty/only 1 ingredient slot is occupied. I wonder if this should be made consistent.