Randomly around the nether in addition of nether fortresses, you could find giant castles and ruins lying on islands at the middle of lava lakes. At the middle, the biggest castle you would have a throne room, over the throne is lying 3 wither skeleton skulls but nothing else, no soulsand, it should be brought after farming nether fortresses as example. So you can spawn the wither, above the chance of spawing ghast is increased, in addition of wither skeletons, witch can also spawn within those ruins. The ruins would be made of stonebrick, netherbrick and some chunk of obsidian. Of course also several mob spawners.
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