If you thought the cast of Captain America: Civil War was massive, wait until Avengers: Infinity War arrives next year. Virtually every character in the Marvel Cinematic Universe (with the exception of the TV-based heroes and villains) has been either rumored or confirmed to appear in the blockbuster film, and it makes sense because it'll take all of their combined might to stop Thanos from gathering the Infinity Stones and ruling the universe.
The first teaser trailer for Infinity War packed a lot of information into its short runtime, so it's little surprise it neglected to show off every single MCU character featured in the film. What we have here is a case of some missing heroes, which makes us wonder, why are they missing? Is Marvel Studios simply playing coy by holding back footage, was there simply not enough time to debut them all, or are their absences a clue that they may not survive Thanos and his Black Order's invasion of Earth?
Ant-Man and the Wasp
The last we saw of Scott Lang aka Ant-Man, he was a prisoner aboard the Raft after being captured in Captain America: Civil War. Luckily for him, the film ended with Cap breaking into the Raft to free Lang, along with Falcon, Hawkeye and Scarlet Witch. Two of these heroes are in the Infinity War trailer (Scarlet Witch sharing a tender moment with a very much human-looking Vision, and Falcon fighting aliens in Wakanda alongside Cap), so where is Ant-Man? A joke can be made regarding Ant-Man's shrinking abilities keeping him from being noticed, but that seems unlikely.
In a fight of this magnitude, Ant-Man will probably want to enlarge himself to Giant-Man status, just like he did in the Civil War airport battle.
As for the Wasp, Hope van Dyne's absence is easily explained by the fact that she's simply not in the film due to Marvel opting to hold her big debut for her co-starring role in Ant-Man and the Wasp. She will, however, feature into the as-yet untitled fourth Avengers film.
As we mentioned above, Hawkeye was also on the Raft during Cap's prison breakout, yet he's missing here as well. His status in Infinity War and Avengers 4 has generated a lot of fan theories, especially after set photos for Avengers 4 showed actor Jeremy Renner wearing a long black coat that covers the majority of his body. This all but confirms Clint Barton adopts the Ronin identity from the comics, where Barton traded his signature bow and arrows for twin katanas in the fallout from the Civil War comic book story arc.
One reason for Hawkeye's absence in the trailer could be connected to his family. If they die during Thanos' attack on Earth, perhaps Clint goes into mourning, only to rejoin the fight in Avengers 4.