Thor: Ragnarok sees the God of Thunder face off against his toughest opponent yet: Hela, the Goddess of Death. Warning: major spoilers ahead. Hela isn't just the Goddess of Death, she also happens to be Thor's surprise older sister, who used her great powers of destruction to help Odin conquer Asgard and the nine realms. Thor defeats her in the end, watching from afar as she goes up in flames along with Asgard, but just because he prevailed doesn't mean Hela is really dead in Thor: Ragnarok.
Like Odin, Loki, and Thor, Hela is a literal god, which means she can't be killed easily. In fact, Thor doesn't even really try to kill her himself. Instead, he resurrects Surtur, a fire demon determined to bring forth Ragnarok (the destruction of Asgard), and allows him to destroy the land of Asgard, with Hela still standing on it. Hela, desperate to rule Asgard, doesn't try to escape, but turns back toward Surtur as Thor, Loki, and the rest of the Asgardians escape into space. In the end, Asgard goes down in flames and then blows up entirely, leaving nothing behind but dust. But the MCU has tricked us before (RIP Agent Coulson — sort of), and a general rule of thumb with movies such as these is not to trust a death until you see a body.
In no scene did the audience get a good look at Hela's dead body or at her in her moment of death. Yes, Asgard was reduced to space dust, but that doesn't mean Hela was too. Her sorcery far eclipsed Loki's skills, and he faked his own death right in front of his brother. That said, it is worth noting that the destruction of Asgard itself could very well be enough to kill Hela, even if we didn't see her physical body die. It's stated multiple times in the film that Hela draws her power from Asgard — the physical place, not the people. She needs to go to Asgard to regain her power and maintain it throughout the film. With Asgard now destroyed, so is the source of Hela's power.
Even if Hela is dead in Thor: Ragnarok, that doesn't mean she's necessarily going to stay that way. As the Goddess of Death, Hela has the power to manipulate the dead. In fact, one of the first things she does upon her return to Asgard in Ragnarok is use the Eternal Flame to bring her army of dead soldiers back to life. Though she is not immortal, her connection with death might give her a certain advantage, like a longer life or a more present afterlife perhaps. Granted, this is all speculation. In the comics, Hela has never been immortal. She has, however, been killed and brought back to life. According to Marvel, Hela had an arc in which Odin killed her to save Thor's life, but brought her back to life "to restore the natural balance and death." So, Hela coming back from the dead would not be unheard of.
Odin died in Thor: Ragnarok, so he couldn't bring Hela back, but Thanos, the villain in the upcoming Avengers: Infinity War, could. Once in possession of the Infinity Stones, Thanos' powers will know no bounds, and there's got to be at least one stone with the power to bring Hela back from the dead. As a man enchanted with Death, it would make sense for Thanos to want Hela by his side. And, considering Marvel Studios got Cate Blanchett for the role, it seems obvious they'd want to bring her back in some capacity. For what it's worth, Blanchett seemed open to the idea, telling Collider, "That's up to the big bosses." Well, Marvel bosses, looks like Hela's fate is in your hands.