The detritus of past industry; The Slag Pits.

One thing you cannot reliably find in the Nether is iron. The Nether definitely has some mystery to it. And it is suggested that this dimension was once something else before; the Bastion Remnants are testament to that. So I would believe that large areas of past construction of these as well as the Nether Fortresses would have taken place. Broad pits of Slag, sand-like blocks, could be excavated and smelted down into iron nuggets. Potentially 3-9 nuggets a block.

An atmosphere of rust and the echoes of industrious machines could be heard while in the pits, suggesting the massive efforts that once too place there. You would want to venture to these Pits in order to mine the residual iron that lays there. And to enjoy a new gloomy ambience of the sounds of machines clanging and molten metal being poured. An offbeat place for a peculiar minded explorer.