Warning: The following contains spoilers for Avengers: Infinity War
Drax has always been one of the more humorous characters in the Marvel cinematic universe; in keeping with the rest of the Guardians of the Galaxy, he's more light relief than hard-hitting drama, but his role in Avengers: Infinity War feels belittled because of it. The MCU has always done well, is to incorporate humor into its movies, but it usually (but not always) finds the right balance between movies that focus more on the plot and characters, such as Black Panther and the Avengers movies, and movies that are light, easy to watch, and more amusing.
Guardians of the Galaxy vol.2 certainly fell into the latter category; it was packed full of comedy sequences and witty one-liners, and Drax was at the heart of that. Played by Dave Bautista, Drax is strong, fearsome, and has a heart of gold. Because his species is literal, it leads to a lot of humor because he cannot understand metaphors. He's also very random in some of the things he does, such as the scene in Avengers: Infinity War when he's eating a bag of chips while standing very still.
There's no denying that Drax gave us some of the funniest moments in Guardians of the Galaxy vol.2, particularly in his friendship with Mantis. It was logical, therefore, that he should also provide comic relief in Infinity War, and the movie certainly needed that change of tone. But it's reached the point now where Drax is only seen as a comedic character, and that undermines the fact that he actually has a heartbreaking backstory.
Drax lost his wife and daughter at the hands of Ronan the Accuser, who was acting on Thanos' orders. Ever since, Drax has vowed to get his revenge on Thanos and although that wouldn't be easy, he's certainly physically powerful enough to give it a good go. The problem is, Marvel has spent so long using Drax to deliver the dry one-liners or the funny random scenes, that his motives have been forgotten and when we're reminded of them, they seem almost inconsequential.
During Infinity War, Drax mentions several times that he wants to kill Thanos, and he tries. The audience, however, is not immediately reminded of his motive, because we're far more focused on what he's going to say or do next. Basically, when's he going to be funny? With Drax now wiped out along with half of the universe, it's up to Avengers 4 to finally convince us that Drax has the appetite for vengeance that we have been led to believe.
One of the ways to do this, should Drax return (and we're assuming he will), would be to have him come back as a much more worn down version of himself. We need to see that Drax knows this is the last chance saloon; that this will be his final opportunity to get the revenge he desires, and we need to see a dark side to him that we've not been shown before. Only then can Drax really be seen as a fully dimensional character who has enough motive and anger to kill Marvel's darkest villain.