Hulkbuster vs. Hulk
Let's get the most obvious out of the way. Hulk vs. Hulkbuster is the showdown that fans have been waiting for since The Incredible Hulk came out right after Iron Man. When Banner revealed the sudden twist, "I'm always angry", fan's hopes and dreams for the colossal brawl shattered.
Enter Maximoff. With the inclusion of Scarlet Witch in The Avengers: Age of Ultron, the possibility blossomed for some reality-shifting chaos. When you look at the Hulk's eyes, it's pretty clear that he isn't quite himself. Those jagged streaks in his eyes are very vibrantly red, or scarlet as some might suggest.
Since it just wouldn't be fair if Banner hung around for the Civil War (he would definitely take his buddy Stark's side, causing the Hulk and the Hulkbuster to be on the same team), it becomes mandatory that the Hulk take a leave of absence. As many fans theorized, all of this Hulked-out destruction could cause the need for Hulk to be shot out into space ([Guardians of the Galaxy](movie:424073) cross-over anyone?). Even if he doesn't get blasted out into the void, something major is going on with the Hulk, and we should all keep an eye on him.
Hulk/[Black Widow](movie:1070824) Love Interest
It seems like the Hulk is going to be emphasized more in this movie than the last one, and this twist is certainly an unexpected one. Natasha Romanoff isn't just the Black Widow, she was trained in a Black Widow program, where she was trained to basically become an unsympathetic assassin. Her friendship with Clint Barton in The Avengers surprised me at first, but when you think about it they both have a lot in common with the whole professional killing thing.
However, a budding relationship with Banner and Romanoff (Banneroff? Romanner?) would be an interesting twist, especially with Banner's violent rampage in the first Avengers, during which he nearly killed Natasha. Even if Natasha and Bruce do end up falling for each other, how is she going to handle it if he ends up leaving to some far-away destination? It seems like this B-story might generate as much interest as the whole legion of robot drones problem.
Infinity Gem Anyone?
The first Avengers film marked the debut of the purple-skinned Titan named Thanos. Since then, the Infinity Gems, or Stones as they are now called in the MCU, have become common knowledge in the films, from the Aether in [Thor: The Dark World](movie:206462) to power stone in Guardians of the Galaxy.
As comic fans mostly know, there are six Gems/Stones. This is the last Avengers movie before The Infinity War, where the Thanos struggle will finally step into the spotlight. Three of the Stones have already been made clear: The Tesseract, The Aether, and the violet gem in Guardians. That leaves three more that Marvel has to cover before the Infinity War. The Avengers: Age of Ultron would be the perfect time to introduce number four.
Fans have been speculating all over the Internet since [The Avengers](movie:9040) regarding Loki's staff. It is said that it is powered by the same energy as the Tesseract (which is a stone), and it had the ability to corrupt the minds of both Dr. Selvig and Hawkeye. This has led everyone to believe that the shiny blue thing in the scepter is the Mind Stone.
Whether it is revealed as an Infinity Stone or not, the scepter was featured in the trailers, which means that Marvel has some major plan for it down the road.
Marvel Has A Plan For Vision
So we all know that Vision is not only going to be in Age of Ultron, but he is going to be a major catalyst at the film's climax. However, Marvel continues to keep the character under wraps. We've seen loads of concept art regarding the brightly-colored android, and we know that he will be played by Paul Bettany (the voice of Jarvis).
Other than the concept art, we haven't got to see a real look at the character. We've seem him in the concept poster in the top left corner and we've seen him among all of the Ultron drones in the new [The Avengers: Age Of Ultron](movie:293035) poster.
With the common knowledge that the character exists, there has to be something big that Marvel isn't letting us in on. Furthermore, will he appear in any future Marvel films? It would be interesting if he fought in the Civil War, but something tells me he would have better things to deal with than human squabbles.
Someone Is Going To Die
In true Whedon fashion, death is all but certain for at least one of Earth's mightiest heroes. As I covered in an article several months ago, I theorize that Col. James Rhodes, AKA War Machine, will be the unfortunate fatality. I based my claim off of the fact that he appears in the trailer alongside the Avengers in Avengers Tower, yet he is absent from all of the battle scenes. Not to mention his death would spark Tony Stark's pro-registration stand in the upcoming Captain America: Civil War.
Hawkeye is another character widely speculated to end up dead in the upcoming Avengers sequel. I personally disagree, as he could be a useful addition in Civil War, but his death is a very real possibility in the film. No matter who dies though, the Marvel Cinematic Universe is going to be drastically changed by this movie, for better or for worse.