Posters for The Suicide Squad have a simple warning; "Don't get too attached." It's something writer/director James Gunn has warned fans about himself, and it's baked into the source material's status as a "supervillain Dirty Dozen." Despite that, it's hard to believe that every character is equally expendable. Given her enduring popularity, Harley Quinn seems like the character most likely to survive her squad's latest suicide mission. As unlikely as it is, Harley's luck could run out sooner than anyone anticipates.
Harley was one of the few DC Extended Universe characters who did not appear in Zack Snyder's Justice League. However, she wasn't completely omitted from the film. During the Knightmare epilogue scene, the Joker brings up the body count of loved ones Batman has racked up, including the Robin Joker killed. Batman retaliates by describing how Harley died in his arms. Before she died, Harley used her last words to make Batman promise to kill the Joker slowly. Batman tells the Joker that he intends to honor that promise.
The scene itself was less about Harley and more about giving fans one scene between Joker and Batman in the Snyderverse. Nevertheless, it does hint that Harley and Batman could develop a similar relationship in the Snyderverse to the one they've had in recent comics, like James Tynion IV's Batman run and the White Knight universe created by Sean Gordon Murphy. It also builds on Harley's dabbling in vigilantism from Birds of Prey, if unintentionally.
In addition, this sets up one potential way for Gunn to shock audiences by killing one of DC's darlings. Batman making a surprise appearance in The Suicide Squad would be, on top of Harley's death, a huge shock.
As bold as this decision might be, it's unlikely to happen for a number of reasons. The main one is that The Suicide Squad is somewhere in between a sequel and a reboot of its 2016 predecessor. If Gunn is making the film accessible enough that audiences don't have to watch the first Suicide Squad, he probably won't be connecting it to the Snyder films, even if they share a cinematic universe.
Whether the Snyderverse is restored or not, Gunn's The Suicide Squad (and the rest of DC's slate of films) seems to exist outside of its confines. That doesn't mean there aren't other ways for Harley to meet her untimely end in The Suicide Squad. The movie's loose continuity could see Harley killed in action but still leave room for her return in other projects, either because of a sliding time line or some other narrative cheat.
One such method could be the Samsara Serum from Adam Glass and Henrik Josson's Suicide Squad run. Made from Resurrection Man's hand, Deadshot and Amander Waller were resurrected using the drug. The serum was later retconned to be potentially fatal to its users when writer Ales Kot took over the series for a short time, but introducing it in the DC Extended Universe could provide an easy way to bring other characters back.
Furthermore, keeping Margot Robbie as Harley Quinn could be a way for DC to embrace the multiverse in its movies, similar to the way it has in its TV shows. Robbie could play a different version of the character in future projects, allowing her to go in different directions than her previous outings.
As many options as DC has to have its cake and eat it too with Harley's fate, the option that makes the most sense is for her to just survive the mission. There's still more for the character to do, in and out of The Suicide Squad, including a potential Birds of Prey reunion. As shocking as Harley's death could be, it's better to save it for Robbie's exit from the role instead of bringing her right back.