Spawners have one key issue: they are easily exploited, especially when you have silk-spawner or BE pickup mods installed. I propose a spawner block alternative that:

• only spawns up to n mobs before breaking
• breaks and is replaced by a loot chest after j mobs are killed
• is unbreakable

This solution would:

• prevent players from exploiting dungeons
• require players to actually kill mobs
• provide a reward for killing mobs
• make dungeons slightly more immersive-- you could find spawners with double the normal cap & spawn rate, but double the loot.