I know that everyone is wondering about every tiny detail of Avengers: Age of Ultron. People want to know every detail down to costume design, but there are probably bigger things we should worry about. After all, we're talking about Joss Whedon here. Remember how we recently found out Falcon will become the next Captain America in the comics, and a lady will wield Thor's hammer also in the comics. If you put that alongside Chris Evans' expiring contract and the introduction of the Winter Soldier, it seems as if they're prepping us for some change. This is not official, it is just a prediction, but chances are the whole world will watch in horror as Captain America will die in Avengers 2.
Every fan boys heart just or will die and maybe that's the reason Joss Whedon might not be involved in Avengers 3. Maybe it will happen early in the film, maybe not. I truly believe that they will follow the Civil War epilogue Death of Captain America story-line in Phase 3. If this is true, that means Bucky will become Captain America for a short period of time. It also means that Steve could be eventually brought back to life in the Reborn story-line, also in Phase 3, maybe 4. There would be an even bigger controversy when comes back to life, hypothetically. It will be the classic new or old debate.
Now, if they really wanted to truly mess people up quite a bit, I have another prediction. Considering our villain is Ultron there may be some betrayal and all. Based on the knowledge we have gained on the basis of J.A.R.V.I.S., Iron Man, and Ultron's connection, we could see Stark's suit taken over by Ultron and potentially killing Cap. This could happen before or after the team discovers the villain Ultron, if it was before we could see Stark becoming an outcast from the team, leaving them out of funding and a base. Stark may have to become the hero again and swoop in just in time to save the day. I personally believe before would be better in a insane type of way. After all, isn't the plan to make a bigger splash than Avengers 1.
This is all just a prediction, some of it may be false, and other parts may be true. Call me crazy, or my theory ridiculous, but no one will honestly know until the movie hits theaters May 2015.