In the Opening Logo Sequence of Avengers: Endgame they used the song, Dear Mr. Fantasy.
Avengers: Endgame Opening Logo Sequence
Dear Mr. Fantasy
What is the significance of using the song?
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Dear Mr. Fantasy play us a tune
Something to make us all happy
Do anything, take us out of this gloom
Sing a song, play guitar, make it snappy
You are the one who can make us all laugh
But doing that you break out in tears
Please don't be sad if it was a straight mind you had
We wouldn't have known you all these years
According to both TV Guide and Movie Web, the song is meant to be about Stark figuring out time travel.
The two sites above, as well as udiscovermusic, have all suggested that the song was one of the mp3's on Quill's Zune and was still aboard the Benetar.
As PopSugar points out, the song came out in the 1960s which is the same decade that Iron Man was first introduced (1963) and starred in his own comic (1968).
In the commentary for the film, Joe Russo said:
We felt like it was correct tonally, but we thought that the lyrics were appropriate for Marvel, and what it’s done and the expectations from giant pop culture films.
and Anthony Russo said:
This being the climactic movie of the 22-movie run, it was nice to comment on our experience of these movies
Talkies Network added that:
the song was meant distinguish the film right off the bat from other Marvel features: