Avengers: Endgame screenwriters Christopher Markus and Stephen McFeely kicked off Hall H proceedings at San Diego Comic-Con 2019 on Friday, reflecting on the two giant Marvel Cinematic Universe chapters they co-wrote — Avengers: Infinity War being the other one — and teasing scenes and concepts that didn't make it to the final scripts. That included Thanos killing all the Avengers in an alternate timeline in Endgame, Doctor Strange invoking the Living Tribunal in Infinity War, and a giant Midgard serpent that Thor fought in Infinity War.
Naturally, the most shocking Comic-Con revelation of them all was the Avengers: Endgame deleted scene involving Thanos. Here's how the idea went: after learning that the Avengers were time-travelling, Thanos from 2014 went to Earth and killed every superhero. He then travelled to 2023 to the Avengers HQ, where the final battle takes place in Endgame, and then presented Captain America with the decapitated head of the other Cap. Markus and McFeely said they were inspired by the Old Man Logan comic, but ultimately cut it because it was too dark. We didn't want to traumatise children, they joked.
On Avengers: Infinity War, the co-writers introduced the Living Tribunal — an entity that oversees and maintains balance across realities, per wiki — because of Marvel Studios president Kevin Feige. This would happen during the fight between Doctor Strange and Thanos on the latter's home planet of Titan, with the former opening an inter-dimensional portal. Thanos would appear in front of the Living Tribunal and be judged guilty (naturally). But Markus and McFeely opted to do away with the scene because they felt it would've been too confusing for viewers.
As for what that Midgard serpent was about, the co-writers revealed that they initially struggled to come up with a decent plotline for Thor in Infinity War. In an early version of the script, Thor would have fought a giant serpent in Midgard to get his hands on Stormbreaker, that Thanos-killing magical axe. But as you can tell, because you're likely laughing or going ‘Huh?', it wasn't working and was axed. Excuse the pun.
Markus and McFeely also revealed that it was Robert Downey Jr.'s idea that Tony Stark / Iron Man wouldn't have any last words as he died. Considering he had prepared that holographic video in the event of his death, it made sense to not take away from the emotion of that moment.
The Endgame co-writers also talked to real physicists to discuss how time travel would work. And at Comic-Con, they showed off images of the whiteboards they used to plan the time heist — and the film in general — during the writing process.