in no particular order:
1. The apocalyptic future that may never happen because the present has changed
Why is it awesome? Because you can have DRAMA!ANGST!DESPAIR!DEATH! without having to have that happen to your characters in the present. This way the show can stay fun to watch:) It can also be a bit of a wild card, throwing random characters together or having characters be completely different. And it can either happen, happen only partially or not happen at all, so you don't have to take it too seriously.
2. Body swapping/ possession/ anything that lets the actor play a different character in the same body
Why is it awesome? Because there's always a bit of a cracky element to it. And when two actors have to play each other, it is ALWAYS fun to watch. And it really tells you who the Great Actors are and which ones aren't.
3. There are many copies/clones
Why is it awesome? It's plain Double the Fun logic. If you have one cool character, two of them is even cooler. If you have one nice character and their evil counterpart, it's interesting to see them interact.
4. Dreams/ hallucinations/ visions/...
Why is it awesome? Because anything can happen. When it's done well it can give us some insight in a character (in a fun way!;) and if not done well you at least have a cracky fun scene:) And unlike AU's, you don't even need to have logic or make up science-y stuff to explain what's going on.