5. Ant-Man
Just like Disney are putting a trailer for Star Wars: The Force Awakens in front of Avengers: Age of Ultron, they would be smart to also include a new look at Ant-Man while they're at it. However, even smarter would be to throw in some sort of reference to the character to really get causal moviegoers and fans excited about Marvel's next "risky" movie. Assuming the events of Ant-Man take place after this sequel, Hank Pym seems like the best option. The trailer indicated that Pym is looking to recruit Scott Lang as the new Ant-Man, so something as simple as witnessing Ultron's attack could be shown as spurring him on to bring back that heroic mantle. Another possibility would be to have Tony Stark meet up with the original Ant-Man to form some sort of alliance ahead of Captain America: Civil War, but however it happens, at least one of the after-credits scenes should be dedicated to him.