It is currently two weeks until the theatrical release of Avengers: Age of Ultron, and boy am I excited. Combined with Ant-Man, these two movies will bring about a close to Phase 2 of the Marvel Cinematic Universe.
Phase 2 has been a wild ride, with Loki seizing the throne of Asgard, S.H.I.E.L.D. falling, and the Guardians of the Galaxy assembling. Some major characters have been introduced, and there are a few that we should definitely be keeping an eye out for.
Remember the guy who was maimed and disfigured at the end of Captain America: The Winter Soldier? That's Crossbones. He's known in the comics for being a pretty brutal mercenary and for having a substantial part in the assassination of Steve Rodgers.
Not only will he probably contribute to Steve's inevitable death, recent plot details for Captain America: Civil War say that Crossbones will cause a catastrophic event that results in the government enforcing the Superhero Registration Act. This Act ultimately is the cause of the Civil War.
So it seems like Crossbones is going to change the whole tone of Marvel movies to come, so definitely watch for him in the upcoming films.
You may know her as Skye from Marvel's Agents of S.H.I.E.L.D. but comic fans know her has Daisy Johnson, or Quake. After the devastating events in recent episodes of the show, Skye's seismic powers are coming in strong. In the comics, she uses these powers to team up with the Avengers, like in the Age of Ultron comic series.
Her storyline has also opened the door for the Inhumans, which is getting a film several years down the road. Perhaps Quake herself will get a signifigant role in the upcoming movie, alongside Black Bolt (hopefully).
As I said above with Crossbones, it is extremely likely that Steve Rodgers, aka Captain America, will be dead by the end of the Civil War. This is when Barnes will get his time to shine.
Just as it what in the Civil War comic event, it looks as if Bucky will pick up the mantle of Captain America after Steve falls. I am actually excited at this prospect. Sure Steve is as much as Cap as Peter Parker is Spider-Man, but a change in casting of such a memorable character is a fresh change.
This means that the Bucky Barnes incarnation of Cap will be the one to take on Thanos in the upcoming Infinity War. Oh, and speaking of Thanos...
Thanos making the list is no suprise, seeing as he is the ultimaye end-goal for the third Phase. He will probably make a few appearences scattered through other films, seejng as yet to locate the final few Infinity Gems.
At the moment, all of the gems (and the Infinity Gauntlet itself) are out of his reach, so expext the Mad Titan to make a few drastic moves to become the overlord that he desires to be.
I could only think of one character to top Thanos on this list, and that would be his daughter Nebula. Technically she is his granddaughter in the comics, but that is just one of the many different es found between the comics and movies.
When Nebula escaped at the end of Guardians of the Galaxy, I knew that Marvel had more important events in store for her. Especially because I know of her fate in the comics.
in one of my favorite events, Infinity Gauntlet, Thanos gains ultimate power by acquiring the six Infinity Gems. He uses this power to kill off scores of heroes and to contort and disfigure Nebula in order to impress Mistress Death. The madness ends when Nebula grabs the gauntlet off of Thanos' hand in order to wield the power herself.
Basically, if things go as planned, Nebula will gain the power to obliterate every living thing in existence. So yeah, watch out for her.
Honorable Mention: Loki
I listed the trickster god under Honorable Mentions because I already listed him under a list similar to this one. That being said, he does have control of the Tesseract and he sits on the throne of Asgard, so watch out for that.