Hmmm, what to do with that 4th empty slot just begging to be utilized....
I know!!
Input/Output Capacity Increase!
Introducing a new Machine Upgrade Card, the Inventory Upgrade...
Made with some very, very, very expensive items. These will allow the machines to have an increased capacity on both the input and output slots, just like the Induction Furnace. Before you say, 'It will make the Induction Furnace useless'; think about the COST associated with the upgrade, it will be enormous.
Simply take one Advanced Machine Block, , and put a Personal Chest on the 4 sides (top, bottom, left and right sides), throw in 2 Iridium Plates in the top 2 outer spots, and finally 2 coolant cells in the bottom 2 outside spots.... and VIOLA, one Inventory Upgrade for your lonely 4th slot.
PS
PS PS
PS
PS= Personal Safe (smileys borked it up)
I don't know, maybe reduce it to Iridium Ore or just one Iridium Plate in the top middle, whatever; it still needs to involve using UU-matter.
Now, from looking at the GUI for the Induction Furnace, it seems that there is room for about 3 inputs and 3 outputs total. This would mean that at most you could add 4 of these upgrades. The initial upgrade (the odd numbered cards) would add an output slot, the following card (even numbered cards) would add an input slot. This sequence would not be user select-able. Also note that EACH upgrade would compound the energy usage, (ie 50% for the 1st one, 100% additional for the 2nd card for a total increase of 150%, 3rd would add 150% for a total of 300%, and 4th would add 200% for a total of a 500% increase in power consumption; OR, if that isn't high enough, start at 100% for the 1st upgrade and it goes to 1000% by the 4th).
NOTE: these are obviously just ideas which the Dev-team could clearly hash out for balance purposes. My goal with these upgrades would be to take the machine to the limit (8000 eu/t) and make the user have to balance the Inventory Upgrades with the Overclocker Upgrades so as to not devastate their energy supply just from running 3 stacks of iron through the Furnace in 64 ticks (3.2 secs)