Little by little, it seems like the themes in the Marvel Cinematic Universe are becoming darker, from what's expected on "Avengers: Age of Ultron" to even things getting heavier during the ABC tie-in series "Marvel's Agents of S.H.I.E.L.D." - and if the "Captain America 3" rumors are correct, things will be getting even darker in the next few years.

The "Avengers" saga is set to see its two films over the summer, with the widely expected sequel "Avengers: Age of Ultron" and then the Paul Rudd-led "Ant-Man," as he plays the next Avenger; however, ever since it was announced that the title for the next Steve Rogers installment was "Civil War," the "Captain America 3" rumors have started making headlines in geek culture websites all over the world.

According to JBG News, the first "Captain America 3" rumors started right after the full title of the movie was announced, as with this came the assumption that the film would be adapting, at least loosely, the widely acclaimed "Civil War" storyline in the comics, which shook up the Marvel Universe at its core just a few years back: in it, the two leaders of the Avengers, Steve Rogers/Captain America (played by Chris Evans) and Tony Stark/Iron Man (played by Robert Downey Jr.) confront over whether superheroes should register in the government, something that would destroy the concept of secret identities.

As Cinema Blend reports, the comic book ark for "Civil War" sees the government issue something called the Superhero Registration Act after unprepared superheroes unwillingly kill 60 schoolchildren, which prompts Iron Man to support the Act and Captain America to oppose it, with different superheroes taking sides, as per "Captain America 3" rumors - in the comics, this ultimately leads to Steve Rogers' death.

Of course, for now the "Captain America 3" rumors are just that, and it is unclear what exactly will be adapted for the big screen and what'll be left out, though star Robert Downey Jr. has confirmed to Fashion 'n Style that the first clues will appear in "Ultron."

In any case, as "Captain America 3" rumors soar, the juiciest one yet says that Spider-Man, the most recent addition to the Disney Pictures Marvel Cinematic Universe after long talks with Sony Pictures, will be making his first appearance in the third Steve Rogers installment, along with Scarlett Johansson (Black Widow) and Jeremy Renner (Hawkeye).