6. "The Other"
It is not so uncommon for the fate of the universe to fall into Fry's hands, and in 'The Why of Fry', the delivery boy was forced to make a tough choice. Never get frozen and finish living his life in his own time, but know that the universe will be destroyed in a thousand years, or let Nibbler freeze him and defeat the giant brains in the future.
Of course, Fry made the right choice, and allowed himself to be frozen again, so he could save Leela from being destroyed. When making his decision, he explained to Nibbler that she was worth saving, to which the creature responded, "she must be 'The Other'".
This clearly seemed like a set up for something of a similar story in the future with Leela. At the very least it left the door open to return to this plot in later episodes, with the audience questioning what this meant. However, the ambiguous conversation was never revisited, going forever unexplained.
Some theorise that it was a nod to Yoda's conversation with Luke Skywalker on Dagobah, but the way it was presented was that there was more story to tell. Particularly with Nibbler telling Fry he would call on him again in the future if they needed him.
Let's just pretend Nibbler never mentioned it then, shall we?