I like reading the more complex suggestions.
Each time a stone is struck with a pickaxe, it has a chance of turning into an impenetrable material that cannot be engraved upon. Stones mined near this new material, Impenetrum, have a slightly higher chance of becoming Impenetrum. Impenetrum is magma safe, and nothing can destroy it, not even demons or Forgotten Beasts or explosives. Thus, eventually, all stone will eventually either be mined or turned into Impenetrum.
Plants are now immune to magma, and Forgotten Beasts have a chance of spawning on the surface when seeds are harvested. Surface Forgotten Beasts could be made of plants, as well as any other FB material. Plant Forgotten Beasts retain the magma safe properties of all plants and while in sunlight, can heal from any damage done to them.
When it rains, there is a chance of Terraria mobs spawning on the map, underground ones would spawn underground, above ones spawning above ground, all Terraria mobs being hostile. Mobs from the Hallow, Corruption, and Crimson may spawn. If any dwarf slays 100 Terraria mobs, shadow orbs and crimson hearts will spawn in the caverns, if there are broken, the respective bosses will be summoned. It is possible to summon multiple bosses without killing the previous, if 6 crimson hearts and 9 shadow orbs are broken, your dwarves will have to deal with 2 Brains of Cthulhu and 3 Eaters of Worlds in short, rain can be a catalyst for much, much more problems.
I guess this thread is more a compilation of both good and bad suggestions. Example, the statues sound like a very interesting suggestion, as does dwarves teleporting bread. Assuming they have no control of where the bread ends up, bread could end up teleporting in front of an arrow, confusing the shooter, the one being shot at, and anyone who saw it