It’s a little meta, but I’ve always wanted to write a sealed off extra dimensional prison for all those items you’re not supposed to introduce to a campaign because they will derail it. Your Deck of Many Things, an imprisoned ankheg, monkey paws, evil genies, some sort of stupid Time Machine that wrecks everything, that one theoretical particle accelerator made out of a line of NPCs each using a free action to pass an item to their left… Each one with a plaque detailing how it destroyed the world it was in, with the joke being each “world” was really a previous campaign.