One was justifying the oddball weaknesses of various creatures -- fairies to iron, werewolves to not having their own clothes and being unable to change back, vampires to poppy seeds (they have an obsessive need to count) or to walls (they need to walk in a straight line), kitsune to virtuous government officials, etc. -- by having a trickster sort of figure offer people powers at an undisclosed prices. The ones that drew iron thought they got the luck of the draw, until the Iron Age. Then, in a fit because they were not so powerful as they were, they forced everyone magical into "safety"; they were still very powerful and managed to more or less force them into hiding. A magical compulsion. An all-pervasive enchantment.
The other was that a mundane dun it. That is, after an ordinary person did something useful -- I think killed a dragon -- he got a wish. And what he wished was that the magical people would stop meddling with ordinary people. And by the nature of the wish, they were magically compelled to enforce it, despite a lot of quibbling about "ordinary."
Which did not explain the randomness. My heroine was, at one point, explaining that there must be rules about breaking the rules, and the weird effects are the side effect of the weird powers, because they aren't actually breaking the rules, they are breaking through to the new, deeper rules. Just as it did not violate the laws of nature that it took longer to boil an egg in the mountains.
But the first idea was not jelling. And then, one day, the muse said take the idea about weaknesses from the first story, and stick it in the second. And there the suggestion sat for days, because my heroine didn't approve. And I realized --
What if the trickster were a legend? She doesn't have to disbelieve. She could think it might be a story invented to explain it. And there could be a dispute about which explains better the lack of new powers -- the trickster went away, or they have not found new ways to break the rules.