Sometimes, a new setting for a story is created not by traveling to a different place in the prevailing reality, but rather by exploring how that reality might differ in another timeline or parallel universe. This can be directly tied to an event in the past being changed by a time traveler, but it can also be allowed to simply exist as a hypothetical. What is your favorite alternate reality scenario in fiction, and why?
Bonus: What is an alternate reality that you wish would have been developed further?
Mirror Universe Bonus: What would the “evil” version of you in an alternate timeline have as his/her/their defining feature? A goatee? Differently-colored hair?