-Make goblin thieves actually thieves - they can now steal items from chests. When they have stolen enough items (about 100, say) they will attempt to run offscreen and despawn, taking all of your items with them.
-Worm enemies destroy blocks when they move through them.
-Undead miners have a chance to carry pickaxes, which allows them to mine through tiles the same way a player does.
-Wood and other flammable substances can burn when near lava. They may also ignite if a flammable block is placed directly above a torch or >3 blocks away from a live campfire. Fire attacks from players or enemies also ignite flammable blocks (like sunfury, flamelash, or fire imp fireballs)
Natural disasters. These will randomly occur and can be destructive to your world.
-Volcano: if mountanious terrain is nearby, it may be labelled as a "volcano" on world gen. It will randomly erupt, spawning lava and falling boulders that explode on impact with tiles, creating bomb-size craters.
-Earthquake: all tiles connected to the ground will be destroyed.
-Tsunami: if you are near the ocean, an animated wave of water blocks may travel over across the shore, flooding everything on the coast.
-Tornado: a tornado (like the Duke Fishron water tornado but white) will appear and slowly move across the terrain, destroying the blocks below it. Nearby blocks will be dislodged and thrown into the air, and when they land, they place as a full block again. If it happens to spawn in an ocean biome, it will be a typhoon, which is only an aesthetic change
-Hurricane: Near the ocean, a hurricane may occur, instantly causing a monsoon event, as well as making blocks randomly break.
...Yeah, stuff like that.