Interstellar
Interstellar deals with the concepts of space and time, which already makes for a pretty confusing film. Cooper (Matthew McConaughey) was pushed into a black hole during a space mission, which put him in a fourth dimension. In this world, he's able to interact with the past.
What Happened to Cooper?
Cooper chooses to tell the past version of his daughter the solution to save Earth. This ended up helping to save the human race, but the question remains: What happens to Cooper in this dimension? It's most widely accepted that they know that they won't have much time left with each other, which leaves a bittersweet ending.