The entire fanbase has thier own opinion on what will happen. Yet I haven't found one that is close to mine.
From only the trailers and my creative imagination to work with, I have developed what I think is a plausible outline of the events in the Age of Ultron.
From the upcoming [Ant-Man](movie:9048) movie and the comics history that Hank Pym is the creator of Ultron. Tony Stark will uncover or reintroduce the Ultron project in some forgotten lab or somewhere. (From the 1st synopsis.. "Tony Stark tries to jumpstart a dormant peacekeeping program, things go awry.") This will name drop Hank Pym and/or Scott Lang to set them up for there debut in Ant-Man. Their movie needs to be part back story and part present day. Ultron's Age will be too big to ignore, something of the lasting effects will have to be addressed in Ant-Man.
After finding this lost tech, Tony would see it as a solution for modern day problems. With possible modifications he polishes it up and stamps his seal of approval with the Avengers logo on the chest plate.
This is introduced publicly and goes south in some way, forcing the team to turn on the newly revealed member and destroy it openly. Afterward, the party scene would take place. Relaxing by themselves they banter casually about Thor's hammer (which is significant to a later plot point.) They are interrupted by they thought defeated Ultron, letting them know that things are only getting started. I say this because of their reactions to seeing him. They don't seem confused at who it is. They turn hostile immediately, Hawkeye especially shows me that the team has already had a round 1 with Ultron at that point. Then Ultrons burst through the walls and...its on.
From here I jump forward somewhat into the meat of the movie, I don't have any theories as to make the beginning link to later details yet. So here goes.. Ultron is constantly upgrading himself to become better. This is a constant thing, (think Tony Stark making new suits all the Freaking time!) So throughout the movie Ultron is becoming a stronger villain. In the comics anytime he is defeated, he corrects whatever flaw or error that led to his shiny ass to be stomped, so that he couldn't be stopped the same way. I think that Ultron will duplicate himself to execute his plans, which according to that first synopsis is human extinction. The many copies become an army that seizes control of the USA. Possibly the world. Of course making the world point fingers at those that failed to stop him the first time (Avengers) once the fighting stops.
This will not be a quick few days of fighting. I think that this movie will take place over an extended period of time. I say weeks if not more. Ultron is a far better villain than any they've faced. Any assault on him is repelled and then the weakness they found is eliminated permanently. The Avengers began and ended in a matter of days. [The Avengers: Age Of Ultron](movie:293035) has to cram in a team reuniting after time apart, a downward spiral, Hydra vs. S.H.I.E.L.D. and to top it all off a rampaging Hulk. Logically this movie can't span just a few days and have the heavy feeling of loss and weariness it should have.
The one line that gives me the most cause to think that it is a long fight is Tony telling someone that "This is the end of the path I started us on." This can mean many things to many people, but I hear that either he blames himself for kickstarting the whole super hero team thing by outing himself as Iron Man in the first place, or more likely that he regrets tinkering with Ultron to begin with led to very disastrous consequences. The background while he says this is alo a clue as the the length of the fight. The are on the run, hiding. Where? The kid's updated lab barn from Iron Man 3. Someplace hidden that also provides tech to work with. If the Avengers have to hide, then the threat must be larger than them.
Black Widow is always the professional agent, I see her as rolling with almost any punch. The world is at peace, then she's on vacation. The world is at war, then she's clocking in overtime. It's all the same to her. But Banner has a connection with her that will be delved into. Maybe her ability to handle any threat makes her well equipped to be a foil to Hulk's rages.
Hydra itself will be a possible villain, but I think that Ultron will handle them rather quickly. What better way to show how big of a bad guy he is than to wipe out the other bad guy in short fashion? The shot of Pietro and Wanda with Ultron in front of them looks a lot like the room that the Hydra agent was walking around in during the post credits scene of Cap 2. I can guess that Ultron learned of either Loki's scepter as a weapon to use against his enemies or was freeing the twins so that they could be his weapons. Why use people to help kill humans? Cause they aren't human? Does Ultron approve of "miracles"?
I would love to see Hydra taken out by Ultron. Leveling the playing field for a power vacuum. He could have his armies storm every cell globally at the same time. This is as close as the comics that movies can get to in regards to moving on to the next Big Bad. Hydra can attack the Avengers in a comic one month and escape. The next month is some other villain with no word of going after Hydra. The MCU has to factor that in, and I believe they have. [Agents of S.H.I.E.L.D.](series:722469) is handling the Hydra threat on the ground. What happens in AoS will set the stage for AoU.
I won't even get into Hulk's rampages. Having Tony don his Hulkbuster armor seems evidence enough that either the suit was a back up plan just in case, or was created out of need.
As far as the final battle goes, Ultron upgrades himself to be better than the weapons used against him, and Vision ultimately takes him out. (Yes I left him out of the plot on purpose.) But before his defeat, each Avenger is beaten somehow. I would say all together, but to give the team cause to crumble, they each need to be personally pulled apart. Banner's face in the trailer certainly shows fear and inner turmoil. Thor's hammer falling makes me think that symbolically he will lose the hammer. Becoming unworthy or worse. As well as choking Tony against a wall shows that all is not well in Avengers Tower. Cap is the spirit of the group, the will that drives them. The shield broken screams out hopelessness. Fighting without hope is useless.
Marvel excels at creating broken people that become great. Now the entire team becomes broken, this is loosely the "disassembled" storyline from the comics. (Which is an amazing read.) And also, from that storyline, a major character dies to help the rest of the team. A small arc of this can be a great lead into Civil War and Ragnarok. As well as a great shake up for Agents of S.H.I.E.L.D.
The end will remind you of the ending of The Empire Strikes Back. It will not be wrapped up happily, loose ends will not be neatly tied. Deaths will occur, chaos will be rampant and nothing will ever be able to go back to normal. But that is what I think makes the MCU great. They can and have changed the status quo so that the next movie doesn't just start from normal and go to chaotic just to get back to normal. They start from the ripples the last one left, make bigger splashes and let the next movie deal with those ripples.
I have many guesses on the post credits scenes, but I'll get into those later. Thanks for reading. Comment your thoughts or objections below.