I'd say a mob that interacts with blocks in a different way. instead of creeper explosion damage or enderman block stealing, it should be a tunneling mob. Like a worm or a zombie miner or something of the sort. It would be rather difficult to kill but its drops would be important for gameplay. Sort of like an overworld wither skeleton in that sense. Also its tunnels would have loot of some source. maybe an ore that only appears around their tunnels.
IDK just ideas
