Thanos was the primary character of Avengers: Infinity War. He was at the very centre of the film and had a mission. He fought against the Avengers and managed to defeat them The superheroes, for the first time in the Marvel Cinematic Universe, were defeated.
The Mad Titan, as Thanos is called, destroyed half the universe’s life just by snapping his fingers. He was wearing the Infinity Gauntlet studded with all the six Infinity Stones which he acquired through the course of the film.
So why is Thanos almost nowhere to be seen in the trailers of Infinity War’s follow-up Avengers: Endgame? All we have seen of him (in the first trailer) is his left hand caressing the crops he has planted at whatever planet he escaped after the snap.
He is apparently wearing the same Infinity Gauntlet that he used to harness the power of all the Infinity Stones. (The Infinity Gauntlet looks destroyed; it is clearly is a single-use device). We also had a shot of his golden armour erected as a scarecrow.
Thanos is not seen in any of the other trailers and TV spots. It is just the heroes eager for revenge and to get their dead friends back. Even in the posters, Thanos is a background presence, and the focus is on the superheroes.
In the Infinity Gauntlet storyline, from which quite a bit of Infinity War’s story came, after the whole shebang, Thanos goes on to live an introspective and quiet life at an unnamed planet (one which is presumably not a ruin like his own Titan). It is Adam Warlock, a fan-favourite character among comic-book readers, who finds him by traveling 60 years in future.
One reason why Thanos isn’t the focus was hinted at by Kevin Feige. He had earlier mentioned that the focus will be on the original Avengers. The film will perhaps serve as a tribute of sorts to them, to give them a perfect send off.
Another reason may be that Marvel has a surprise in store for us. It is unlikely it could top Infinity War, though. The surprise could be an unexpected villain, somebody even stronger than Thanos. Perhaps some cosmic power who was manipulating Thanos. It could be Death or Lord Chaos, both of which are actual Marvel characters. Or perhaps Galactus, a cosmic being who devours entire planets. Galactus was owned by Fox, but now with Disney’s Fox acquisition, Galactus can be used in the MCU.
Thor almost killed the Mad Titan in Infinity War. Captain Marvel is said to be the most powerful superhero in the MCU and we can probably presume facing Thanos would not be too difficult for her.
With the Infinity Gauntlet destroyed, Thanos cannot use the Infinity Stones again without reforging another one.
So either we are going to see another Big Bad, or Thanos is going to get a power boost somehow.
Avengers: Endgame releases on April 26.