Marvel's Timeline Is Broken Anyway
Before going into the time travel pitfalls, it's first worth noting that the Marvel timeline we're dealing with is rather complicated. The shared continuity has been a key selling point for the MCU, allowing movies to effortlessly cross-promote and a wide ensemble to be called upon at any moment, but in terms of cohesion, it's usually only in a macro and character sense. You can place the films in order, chart the heroes' arcs, and big events will be mentioned in various films, but if you dig deeper into throwaway references and Easter Eggs, things don't line up anywhere near as well. The MCU is so big that its micro continuity just can't work.
The most egregious example has to be Spider-Man: Homecoming, which announced itself as set eight years after The Avengers, despite all in-universe logic suggesting it was only four years on. This was eventually reversed by Avengers: Infinity War, which through carefully placed lines of dialogue essentially retconned the "eight years" text card, but even with the fix you have a very unclear timeline.
Indeed, in the nine months between Spider-Man: Homecoming and Avengers: Infinity War, there were plentiful fan solutions provided that highlighted further inconsistencies, as well as two subsequent releases - Thor: Ragnarok and Black Panther - which altered events of previous films in their own, little ways. The MCU timeline works for the purposes of making new movies with a shared history; like the previously discussed movies, they succeed in their primary purpose. It does not, though, if you're going to work back through it with any sense of consistency.
That makes time travel immediately complicated - how do you balance out which events happened in what way? - but is made all the trickier when you actually dig into potential mechanics.
Avengers 4 Is Destined To Confuse
Avengers 4 appears to be going for the rewrite approach, with the surviving heroes working their way back through time in a bid to take the Infinity Stones before Thanos ever got his golden mitt on them. We already know from set photos this involves alternate versions of events like Ant-Man at the Battle of New York, meaning regardless of eventual success, time is rewritten in some small way; an impact must be felt. Add that we have two different methods of time travel - the Time Stone and the Quantum Realm - and it's all a bit unwieldy.
It's definitely possible that Christopher Markus and Stephen McFeely have done the impossible here and managed to introduce, develop and maintain mechanics that are consistent throughout the film and leave the MCU still working forward come Avengers 4's end. Again, this won't be the ultimate mark of the movie's success - Avengers: Infinity War shouldered comparable logic leaps - but definitely be will something watched for as the story unfolds. However, even if they do make it hold up to scrutiny, that's no guarantee the movie will be understood.
And that is the real barrier to making a coherent time travel movie; not only do other films tend to conflate different methods, but so do audiences. You need only look at the likes of Lost and Harry Potter and the Prisoner of Azkaban to see this in action; both had clearly-defined and stringently followed methods of time travel - "whatever happened, happened", with characters able to jump to the past but incapable of altering anything - yet fans to-this-day question why being a part of Dharma didn't change the present or why Harry didn't just kill Voldemort in the crib. None of the confusion really comes from the media itself, but an audience bred on time travel confusion. Little wonder Deadpool 2 didn't even try to make sense of Cable's watch.
For something with as big and varied a fandom as the MCU, this will inevitably be repeated. Time travel is inherently confusing and there's nothing about Avengers 4 to suggest it will be any different. The only real way out is to make sure there's more at play than just a journey back to collect the Infinity Stones.
- The Avengers 4 / Avengers: Endgame (2019) release date: Apr 26, 2019
- Ant-Man & The Wasp (2018) release date: Jul 06, 2018
- Captain Marvel (2019) release date: Mar 08, 2019
- Spider-Man: Far From Home (2019) release date: Jul 05, 2019