What about a Furnace-type GUI? (with explanation)

This would require two things. A processing duration for the input to turn into the output, and a fuel system. Preferably seperate things.

PROCESSING: This is simple enough, it's just an output timer. Let's say that input(s) X equals output(s) Y. So using this, if X is in input slot(s), wait a specified amount of ticks, then consume one X from the input slot(s) and output one Y in the output slot(s). Adding this to a GUI would of course also add the progress bar seen in Furnaces to the GUI. The icon doesn't have to be customizable.

FUEL: Also simple enough. First, there is no need for custom fuels on the first itteration of this. That could be added later. So just use Furnace fuels for now. Let's say that input(s) X equals output(s) Y, and Z is the fuel. So using this, if X is in input slot(s), nothing happens. There is no output(s) unless the machine's fire is burning. So if Z is in the fuel slot, and X in the input slot(s), one Z is consumed and sets the fuel timer to whatever that specific fuel sets the timer to. Only during this duration can Y be outputted. This would of course be compatible with the processing timer above. The timer only ticks down if the fuel timer is active. In essence then, the rest of the GUI is disabled unless the fuel timer is ticking. Adding this to a GUI would of course also add the fuel progress bar seen in Furnaces to the GUI. This icon also doesn't have to be customizable.

(EVENTS: It would also be nice to have "When fueled..." and "When processing..." events, so you could add some particle effects or sounds or something to show on the outside that the blocks are processing something/are fueled.)

Thanks for reading, if you did. I hope to see this implemented soon.