It would be nice if there was a basic furnace that could be made with just stone and coal and would only be useful for making bronze. That way a Blast furnace made with Surtling cores would be an upgrade, maybe even make it use a little less coal per ingot due to more efficient flames or something. Heck maybe there could be a basic furnace which only does 'basic' ingots, copper, black iron, silver. And the Surtling furnace can do alloys and basic ingots so bronze, Damascus iron, Electrum silver or whatever.

Edit. Also if they do add a basic furnace and make the Surtling furnace capable of making more powerful stuff then they would have to make Surtling cores rarer, maybe larger more dangerous dungeons? Make Surtlings rarer and more dangerous themselves, maybe even needing special means to kill rather than the normal whack em with a stick strat?