From Hawkeye's hair to the new space suits. https://t.co/WpnvmdqINu— Esquire (@esquire) March 15, 2019
Including things several ONTD-ers have speculated about.
It looks like Clint is training his daughter, will she be the future Hawkeye?
“Whatever it takes."
The theory on Reddit is that the four Avengers say this line: Captain America, Black Widow, Hawkeye, and Iron Man, will be the ones to sacrifice themselves to save the world.
Possible time travel and/or time skips in the story.
Based on the several hairstyles The Black Widow has in the trailer.
The Avengers matching suits may hint at time travel.
They appear similar to the suit Ant Man wears which allows him to travel into the Quantum Realm and jump through time pockets.
The red in the flashbacks could be a hint about the red Infinity stone, which is the Reality Stone, factoring into the story. Or it could just be a Red herring.
Sources: the Esquire tweet Esquire Buzzfeed