My own take on the matter:
-Harley Quinn: 100% safe. Does not need explanation.
-Deadshot: Arguably *the* major character of the comic, cast as Will Smith, and framed as the male lead of the movie. The only way he dies is if WB doesn't want to pay for Will Smith more than once, but Will Smith probably doesn't cost nearly what he once did. Almost as safe as Harley Quinn.
-Rick Flagg: 50/50. He really *should* be safe, given he's a major character in the comic, and logically the team needs to have *someone* like him leading it, so killing him off would be stupid and pointless. Plus, he was originally going to be played by Tom Hardy. However, its plausible they've decided to make Deadshot the team leader in the future ( illogical as this is ), and his relationship with Enchantress puts something of a death pendulum above them both. Plus, he's the boring good guy in a movie of charismatic villains.
-Killer Croc: As a member of the Batman rogues gallery, and one with some decent name recognition? I can't see him being killed in a non-Batman movie. Him being killed would surprise me almost as much as Deadshot.
-Katana: Toss up. On one hand, she's technically a borrowed character, and one loosely aligned with the Bat universe to boot. She's also more distinct and harder to replace than Flagg, in terms of Token Good Guys. OTOH, she's not exactly popular and well known, and if they want to kill off any female team members, there aren't a lot of options.
-Captain Boomerang: He *should* be safe, as he's both a borrowed villain, *and* a major team member in the source material. However, given Flash isn't exactly a moving rushing to release lead by a high prestige director, I am a lot less certain about his protection. Especially given his schtick is a little silly.
-Enchantress: Really depends on how exactly she's used in the movie, but if its true she spends at least part of the film as the antagonist? I'd say her odds aren't great, especially with the relationship death pendulum. If both Flagg and Katana live, she almost certainly dies, and if she lives, at least one of them will die.
-El Diablo: Guy is probably doomed. He's a complete obscuro no-name, whose role in the comics was "casualty". Only thing possibly sparing him is a cool schtick and that the teasers suggest his presence in late movie scenes.
-Slipknot: Guy is *definitely* doomed. He's a complete obscuro no-name, with a fairly lame schtick, whose role in the comics was "comic relief cannon fodder", and has trailer scenes that look a whole lot like an outline of his death. Only way he could be more doomed is if they showed his head literally exploding in the trailer.