Last time we saw Heimdall was during Avengers: Age of Ultron, in Thor's vision, when he was blaming the God of Thunder for deserting Asgard and thus dooming it. It was a key scene of that movie, since it showed Thor that greater powers were at work and that the entire universe was in peril. In that vision, Heimdall and many other Asgardians seemed to be in Hel.
But you already knew this. What follows might be considered a spoiler.
According to Latino Review, the Asgardian was indeed in Hel. The website reports that it was all Hela's doing, the villain rumored for so long to appear in the MCU. In Thor: Ragnarok, of course, the God of Thunder will try to prevent that vision from actually happening. But he will fail miserably.
Against the combined forces of Hela and Loki, he will stand no chance, being severely beaten, having his hammer taken away from him, and being banished from Asgard. This isn't all, though, since Heimdall will be killed when coming to his aid.
But Idris Elba's role won't end there. According to Latino Review, the final act of Thor: Ragnarok will feature a battle between the villains (and their army of undead) and the united forces of Thor, Hulk, the Valkyries (who haven't appeared until now in this shared universe), and the souls of the righteous fallen. So, even if for just a single battle, Heimdall will be resurrected.
Among the many Thor characters presented so far, Heimdall seems to have been the most underused. One of the most powerful Asgardians from the comics, as well as the trustworthy guardian of Asgard, wasn't even able to see, in the movies, a giant spaceship behind him. It is about time he had a piece of the action, where he can demonstrate why he was chosen as a watcher.
Of course, these all should be taken with a grain of salt.
But what do you think? Will the last Thor movie finally do justice to Heimdall?