TL;DR
Research to speed up Furnaces, similar to mining productivity.What ?
Integrate Research to speed up Furnaces, to balance mining productivity.Similar to mining productivity, it could be an infinite research.
In the Research tree it would be locked behind advanced material processing wich gives electric furnaces.
Similar to mining productivity a 2% speed increase per tier could work.
Similar to mining productivity it would just give a permanent 2% speed, energy consumption and pollution increase.
Why ?
Furnaces should be able to scale with your Game Progression and currently Steel/Electric Furnaces really don't cut it.I have a medium size Factory currently on mining productivity 62, so my miners produce 2,34 times as much as "normal" miners.
That means I can have more production with fewer miners, but the Smelting Arrays become insanely big. Just to have a modest 600 Iron/s you need massive Furnace Columns wich look out of proportion to the tiny mining fields. Especially since Electric Furnaces are used even more since Energy has become essentially free with nuclear, making the smelting 1,8x bigger.
I would love to have my well proportioned Factory back where Furnace Arrays were big, but not ludicrous.
It would also allow for easier upgrading of existing Factories, where you can upgrade the Mining, Belts and Factories with modules, but have to squeeze in more Smelting since you can't get faster than Steel/Electric Furnaces.
I think it would make Factorio a better Game because all of Factorio is about Balance, and having minuscule Mining feeding MASSIVE Smelting definitively doesn't feel balanced.