New Avengers

Captain America (Steve Rogers)
Marvel's most famous patriot, the man who socked Hitler in the jaw and whose return signaled the end of Marvel's Dark Reign.
Is he marked for death this week? In that he's literally just finished a series called Reborn where he came back from the (assumed) dead, of course not.

Captain America (Bucky)
The replacement Cap, Bucky Barnes is a former sidekick turned cybernetic brainwashed assassin turned American hero.
Is he marked for death this week? There may not be a place for him now that the original Captain America is back, but as one of the few genuinely successful "new" characters Marvel has created in the last decade, he's probably safe for now, and likely to find a new identity at some point soon. Plus, they've already killed a Captain America.

Luke Cage
One-time Power Man and Hero-For-Hire turned moral center for the Marvel Universe, Marvel's former Blaxploitation cash-in has become an important figure in the Avengers books in the current era.
Is he marked for death this week? Probably not; apart from the fact that he has a new solo series beginning in April, one would hope that Marvel and writer Brian Michael Bendis would remember the calls of racism that accompanied the death of Black Goliath in Civil War. Oh, and he's just had a near-death experience in New Avengers, as well.

Ms. Marvel
Marvel's poster-child for "overdoing the personal problems aspect of the character" - She's an alcoholic who's been raped by her own son, had her memories erased and life stolen on two different occasions - and "creating female versions of male characters," but she does hold the honor of having been created by Stan Lee.
Is she marked for death this week? Possibly. Her own series has just been canceled, which would allow for her demise, and she's enough of a B-level character that her death would have dramatic impact without overshadowing the rest of the story. But would Marvel really begin their "Year of Women" by killing off one of their most high profile female characters?

Mockingbird
Former spy turned superhero, Mockingbird has only recently returned to the land of the living after her death was retconned in Secret Invasion.
Is she marked for death this week? Unless someone at Marvel really hates the character, probably not.

Ronin
Longtime Avenger Clint Barton, who's most well known as Hawkeye (but also spent some time as Goliath). A bedrock of Avengers mythology.
Is he marked for death this week? I'd have said "Definitely not," but Kevin Melrose has pointed out that the Hawkeye that appears in the Heroic Age teaser image looks... wrong:

I'm no robotics, or prosthetics, expert, but that left arm certainly appears to be bionic. Just like a certain sidekick-turned-freelance assassin-turned-Sentinel of Liberty. It's now guaranteed AN AVENGER WILL DIE!! in next week's Siege #2. My money is on Clint Barton — again — with Bucky by the end of the miniseries or the beginning of the new Avengers title, I don't know, honoring his memory by donning the purple-and-blue tights. (For the record, the Marvel website says Clint has blue eyes; Bucky has brown. Guess the eye color of Hawkeye in the promo image. C'mon, guess.)

If that's true, then I give up; Siege's writer Brian Michael Bendis has already killed Barton once before, in the Avengers Disassembled storyline that started the massive arc that Siege is, apparently, finishing. But when does thematic consistency become repeating yourself?

Spider-Man
Your friendly neighborhood mega-franchise wallcrawler.
Is he marked for death this week? Oh, come on.

Spider-Woman
Super-spy and former private detective, Spider-Woman is Marvel's second most high profile female knock-off of a male character (behind She-Hulk), and a particular favorite of Siege and Avengers writer Bendis.
Is she marked for death this week? Very unlikely; she has her own series and she's a relatively new character to the Avengers books, after it was revealed that the majority of her previous appearances were actually an imposter.

Wolverine
He's the best there is at what he does, and what he does is very lucrative.
Is he marked for death this week? See Spider-Man.