What do Dr. Horrible's Sing-Along Blog, Serenity and The Avengers all have in common? Outside of adorable casts of characters, it's the sadness and shock associated with the death of a key character in all of these Joss Whedon projects that fans latch onto the most.

For Whedon's next big film endeavor, The Avengers: Age of Ultron, the idea of "death" has been teased as far back as February, long before the title of The Avengers 2 was revealed. Is it therefore a forgone conclusion that one of Earth's Mightiest will not live through the next team-up event?

Speaking with EW about Marvel's Agents of S.H.I.E.L.D. series airing on ABC next month - a show where oddly enough one of Whedon's victims is resurrected - the writer/director and overseer of the Marvel Cinematic Universe spoke about idea of legitimately killing off a key character from the Marvel library.

"I'm always joking about that. Um…maybe?... But I'd have to have a really good reason, a really great sequence for [Marvel executives] to go, 'We'll cut off a potential franchise, that's fine!' They know as any good studio does, that without some stakes, some real danger, how involved can we get? We don't just rule it out across the board, but neither is the mission statement 'Who can we kill?' We try to build the story organically and go, 'How hard can we make it on these people?' You go to the movies to see people you love suffer-that’s why you go to the movies."

With at least two new characters joining the team in the brother-sister act of Quicksilver and Scarlet Witch, possibly played by Aaron Taylor-Johnson and Elizabeth Olsen respectively, and contracts not getting any longer on actors playing already-introduced characters, Marvel will have to find a creative way to remove and replace characters on the core Avengers roster in the long-term. Whether that happens in Age of Ultron or sometimes further down in Phase Three of the MCU remains to be seen, but from the get-go Whedon has been clear about making his sequel more personal, more gripping and more - wait for it- "dark."

Hawkeye Cover Marvel Comics Now

Lately it's been repeatedly mentioned by Whedon and Marvel Studios President of Production Keven Feige (the man behind the curtains of the entire franchise) that Clint Barton aka Hawkeye (Jeremy Renner) would get plenty of screen time in the sequel after practically being written out of The Avengers by becoming a brainless minion of Loki. With the actor's schedule, Hollywood demand and minimal potential for spinoff, the ace archer is at the top of our list for potential to-be-KIA heroes.

There's also potential for one of the new characters to not make it past their introduction and we suspect Scarlet Witch could very much be in danger as well, given that her character's death is the primary plot motivator of the third volume of The Ultimates in Marvel Comics (which also happens to feature Ultron), a series of books the film universe draws much inspiration from. There's also the fact that she will potentially be the first Avenger we meet whose powers are based on magic and that's a can of worms Whedon may not want to keep left open for Avengers followups if she's too powerful.

We can't talk about character deaths without mentioning Captain America. At the end of the Marvel Civil War comic book event in the comics, Steve Rogers was assassinated, leading to his old pal Bucky (The Winter Soldier) picking up his shield and taking over the role of Captain America (until Rogers eventually returned from the dead, of course). While we don't know if a Civil War story is possible in the film universe due to the rights of many key characters having been spread among other studios, the idea of Steve Rogers dying is one Marvel might be crazy to ignore, especially with Chris Evans negotiating his initial offer down from nine features to just six, and the fact that The Winter Soldier is going to be introduced in the Captain America sequel next spring (played by Sebastian Stan who also has a six-picture deal).

After bringing back Agent Coulson, would the next "death" need to be for good or does such a concept not exist in Marvel comic book films, much like it doesn't in Marvel Comics?

 [poll id="667"]

Thor: The Dark World on November 8, 2013, Captain America: The Winter Soldier on April 4, 2014, Guardians of the Galaxy on August 1, 2014The Avengers: Age of Ultron on May 1, 2015, Ant-Man on November 6, 2015, and unannounced films for May 6 2016, July 8 2016 and May 5 2017.

Follow Rob on Twitter @rob_keyes for your Marvel movie news!

Source: EW (via ComicBook.com)