Just an idea, maybe there could be a lava lake, forming from maybe a crack in the ground or maybe even a mini volcano. This lava could be farmed (bucket, new recipe?) but would empty after a few bucketfuls. After maybe a about 2-3 days the crack/volcano gushes out lava again, this would make a deep BOOM noise and make the ground shake a bit so you know. When this happens a few boulders are spewed out in the near proximity, this would also make people actually farm the lava, so they work for the boulders because they are rather rare in the later days. (Just an idea, these boulders would be slightly darker than the average boulder and could have the chance of dropping obsidian, or something similar, when mined). Also lava/obsidian could be used for new weapons/armour/alchemy overall new recipes. Sorry for the wall of text and leave some feedback on what you think!