Dungeons in caves. I guess it could be done as part of a bigger cave update if necessary but the game has all these big and impresive places where you have a challange if you want the loot (Guardian Temples, Nether Fortresses, End Cities and the new Bastions in 1.16). But since they were added in the early alpha/beta they haven't changed. I think they need to be made a bit bigger than a small box where the spawner is made useless by a low tier pick or torches and often contains top tier loot.
