The Meaning of Life, the Universe, and Everything.
I think there should be a nbt tag that can be applied to all humanoid mobs (Zombies, Skeletons, Witchs, etc.). That would allow them to break blocks when trailing another entity that either attacked them or something they naturally attack. They would not spawn normally, only with commands. Though it would be incredibly useful for creating, mini games, custom maps, and just making survival harder (Though this would require a little bit of command block tampering.). If somebody applies it to survival it would make it so they aren't able to just sit in their base (Unless its floating or underwater). Or imagine a survival where once you build a base, every night zombies attack your base and you have to build walls iron golems, snow golems, lava pits etc. Or perhaps a game mode where some enemies can break blocks.