Avengers 2: Age of Ultron
A full concept art poster for Marvel's 'Avengers: Age of Ultron.'Marvel

New speculation and rumors have surfaced about next summer's Avengers: Age of Ultron movie.

According to reports, Avengers 2's end credits may introduce two new characters who are set to make their debut in a future Marvel movie: Captain Marvel and Doctor Strange.

The official synopsis of Avengers 2 from Marvel reads:

Marvel Studios presents Avengers: Age of Ultron, the epic follow-up to the biggest Super Hero movie of all time. When Tony Stark tries to jumpstart a dormant peacekeeping program, things go awry and Earth's Mightiest Heroes, including Iron Man, Captain America, Thor, The Incredible Hulk, Black Widow and Hawkeye, are put to the ultimate test as the fate of the planet hangs in the balance. As the villainous Ultron emerges, it is up to The Avengers to stop him from enacting his terrible plans, and soon uneasy alliances and unexpected action pave the way for an epic and unique global adventure.

Marvel's Avengers: Age of Ultron stars Robert Downey Jr., who returns as Iron Man, along with Chris Evans as Captain America, Chris Hemsworth as Thor and Mark Ruffalo as The Hulk. Together with Scarlett Johansson as Black Widow and Jeremy Renner as Hawkeye, and with the additional support of Samuel L Jackson as Nick Fury and Cobie Smulders as Agent Maria Hill, the team must reassemble to defeat James Spader as Ultron, a terrifying technological villain hell-bent on human extinction.

Along the way, they confront two mysterious and powerful newcomers, Wanda Maximoff, played by Elizabeth Olsen, and Pietro Maximoff, played by Aaron Taylor-Johnson, and meet an old friend in a new form when Paul Bettany becomes Vision.

In addition, the plot reveals that Avengers 2 will reportedly be "much darker" than the first film.

BadassDigest has offered new angles and a few interesting details on how the sequel's direction could take a new turn, and according to the website Captain Marvel might sneak in during Age of Ultron's ending credits.

Captain Marvel
Captain Marvel may feature in Avengers 2 end creditsMarvel Comics

The website speculated, "At the end of 'Age of Ultron' Cap will assemble a team that is a mix of familiar and new faces, leading forward into Phase Three. It's an exciting mission statement for Marvel Studios, letting audiences know that the Avengers team is more than just the guys who were in the first film. The make-up of that team is, technically, up in the air until that scene is shot (I suspect it's being saved for either scheduled reshoots or will be snuck into the schedule of Captain America 3, which goes before cameras early next year), but Joss Whedon's script did indicate who would be on that team. I won't reveal all the names – and some have changed in between drafts, by the way – but one character who has always been in this new team is Captain Marvel."

Other than Captain Marvel, Doctor Strange has also been rumored to make his big-screen debut at the end of the blockbuster sequel. Joaquin Phoenix is rumoured to be playing the lead role as the surgeon-turned-magician, but the Marvel studies is yet to make an official statement on casting.

The Avengers: Age of Ultron trailer is rumoured to be out next week, and is set for release on 1 May 2015.