What if the Freneskæ Creator God, in some hissy-fit of divine wrath, once punched the world in ire, causing its extreme volcanism by fracturing the planet's crust into innumerable pieces? On Earth, we have most of our plates the size of multiple nations. Imagine how many volcanoes would form if there were not dozens of tectonic plates in the world, but thousands?
What if Moonlight Mead were actually like mouthwash to the werewolves? It could explain the feeling of "something dying" in your mouth when you drink it: the bittercap mushrooms used in its brewing make it a great sanitising agent.
There's not likely ever to be a real explanation for it, but how is it that most races throughout the multiverse all speak the Common Tongue? I can see it being widespread throughout Gielinor, as human influence grows and all that, and with it our language. But I do think it'd be cool to have a sort of Tower-of-Babel-in-Reverse type of story somewhere which explains such a convenient plot device.
I kind of have this image in my head of there being an Aztecy/Mayanish civilisation on Karamja in the First or Second Age, prior to the coming of the Karamjan God, which worshipped the Dragonkin. It could explain the statues of Brimhaven Dungeon (more interestingly than just "Sakirth waz here"), and also the Agility Arena. Plus, human sacrifice, and ziggurats.