4. Steve Rogers & Peggy Carter - The Avengers
One of the most tragic sub-plots in the entire MCU so far is the blossoming love story between Steve Rogers aka Captain America and British agent Peggy Carter (Hayley Atwell), which was brought to an abrupt halt when Rogers crashed his plane into the Arctic, and is resurrected 70 years in the future, by which point Peggy is, of course, a very old lady. It seemed certain that, in The Avengers, a great way to develop Cap's character would be to have him go see an elderly Peggy at the end of the movie, but alas, it didn't happen. To be fair to Whedon, The Avengers was already an extremely long movie as it stands, so it's hard to be too mad about it. Plus, set pictures ensure that this is going to be addressed in the upcoming sequel, Captain America: The Winter Soldier, as we know for certain that Hayley Atwell appears as an old lady. Still, it's all about the execution, so we'll have to wait and see...