I've never seen anyone do this but you can farm mithril. I think.
That is from the mithril ingot page. Barrow-Wights could be farmed, I haven't tried it. It says that Barrow-Wights spawn randomly on unlit grass blocks in the Barrow Downs. They will also target any NPC. So, set up a spawning platform of unlit grass blocks, light up other blocks to concentrate spawning to that area, and use a NPC as bait by placing them in the center so that the wights walk off of their spawning platforms and fall into a killing chamber where you can farm them. Would that work? I'm planning to test it in creative soon.
Edit: I think you could farm Utumno orcs and Tormented Elves both at once on the obsidian level of Utumno using a similar trick, but it would be a lot more difficult.