Would love if there were some ancient mines that go deep down into the mountain, and then deep down into the earth until you get to magma level. There would be a big ancient locked door at the very bottom, and you get a key to enter the "Molten depths" once you defeat Yagluth. There would be many pale-skinned, small trolls who are miners and live off of glowing mushrooms and eat giant spiders for food. These small trolls would be enemies with the fulings too, and sometimes small patrols of these smaller trolls would be seen leaving the mines and heading towards the plains. They are immensely grateful for Moder and her drakes, which while hostile to them, usually deal with any invading fulings or other creatures. The mines wouldn't be at the edge of the mountain, but near the middle.
The molten depths would of course have plenty of lava, and have ancient ruins where the dwarves once lived, which could be the secret ruins of Svartelfheim (real place in north mythology, it was the underground system of mines and caves where the dwarves forged their legendary equipment, such as Thor's hammer and Odin's spear.) which was hidden in Valheim where Loki would least expect it. Upon finding out it was in Valheim, Loki meddled with the place to anger his family for keeping the secret from him. He raised the level of lava, burning most of the dwarves alive, and making the place too hot for any creature that is not a lava blob, magma golem, fire wizard or ancient fire spirit to live in. The dwarves that did escape were cursed by him and turned into small trolls (small trolls are a real thing in Norse mythology, just like the big ugly ones.) so that they would never be able to craft their legendary weapons again. He handed the key to Yagluth, arch enemy of the dwarves, so that it would be kept well and used to torment them. The fight between these two would mirror the much larger fight between the black and white elves, which Loki found amusing, as he could be in control of it and mess with both sides.