The Marvel Cinematic Universe has been largely commended for the way it was weaved together a series of nearly 20 movies into a cohesive tapestry that uses multiple characters and numerous storylines, but brings them all together into a single world that feels alive and thriving. For the most part, the MCU deserves the credit it has received for this unique achievement, but let's not pretend they've been perfect.
While many of the MCU storylines have paid off in big ways over the course of several movies, there are others concepts, characters, and plots, that were clearly set up to mean something once have been seemingly forgotten about. With Phase Three of the MCU coming to a close and many characters likely leaving the MCU forever, there's a possibility that Avengers 4 could be the last chance we have to get these plot threads tied up.
Where is Lady Sif?
Thor: Ragnarok completely destroyed Asgard, and Thanos has now cut the population of the remaining Asgardians in half. All of the Warriors Three were unceremoniously killed by Hela, and Odin died of natural causes. We've seen the fates of most of Asgard, but Lady Sif is nowhere to be found. Was she on Asgard when Hela started killing people, did she make it on board the ship to escape the explosion? Was she killed by Thanos? Has she been hiding someplace else this whole time? We have absolutely no idea what's going on with her and we'd like to know. The remaining Avengers could certainly use whatever help they could get.
What Happened to Betty Ross?
The Incredible Hulk is something of the black sheep of the Marvel Cinematic Universe; nobody seems to like to talk about it much. But when William Hurt returned as Thunderbolt Ross in Captain America: Civil War, it made it clear that we're not pretending the movie never happened. While Bruce Banner's relationship with Black Widow seems to be very much up in the air, it makes one wonder what happened to the woman he loved first, Betty Ross. We're not saying Liv Tyler needs to appear in Avengers 4, but some explanation about why they apparently don't see, or even speak to each other, anymore wouldn't be out of line.
What Happened With The S.H.I.E.L.D. Leaks?
The big twist back in Captain America: The Winter Soldier was the discovery that not only had Hydra never been destroyed, but they actually had continued to exist from within S.H.I.E.L.D. In order to reveal Hydra's existence to the world, Black Widow ends up dumping everything S.H.I.E.L.D. has, declassifying the entire database. We know that Zemo got his information on Bucky from the database, but there has to be a lot of useful information there that could be used by anybody for good or ill. What has the rest of the world been doing with all that information? Is there anything on Carol Danvers in there?
Ok, this one isn't strictly an open plot thread for us. Hardcore fans have seen Agents of S.H.I.E.L.D., so we know what happened to Agent Coulson, but the rest of the Avengers don't. With the character returning to the silver screen in the upcoming Captain Marvel movie, it will remind fans of the character right before Avengers 4 comes out. As such, it seems like the next Avengers film would be a perfect time to let the cat out of the bag. You know Captain America has been feeling like shit for not treating Coulson better ever since he "died." Relieve the poor guy's guilt. Again, we don't need Coulson to appear (though it would be awesome if he did), just provide the Avengers with the information. They can read Nick Fury the riot act for not telling them the truth later.
Where Is Samuel Sterns?
At the end of The Incredible Hulk, we see the character of Samuel Sterns being infected by some of Bruce Banner's blood, which, as comic fans are aware, will turn him into the super intelligent being known as The Leader. Unfortunately, we haven't seen or heard a word from Sterns since then. While this would be an odd character to bring into Avengers 4 out of the blue, we would have said the same thing about Red Skull, and here we are. Under the circumstances, a super intelligent being might be quite useful, and maybe the MCU version of the character is somewhat less evil than his comics counterpart.
What's the Status Of The Nova Corps?
This one isn't a long-standing plot thread, as it's one that has been left dangling by Avengers: Infinity War itself, but we learned there that Thanos took quite a toll on Xandar when he attacked it to get the Power Stone, but what does that mean for the current state of the planet and the Nova Corps? Are they completely destroyed? Did some survive? One would bet they'd love to get some payback if the Avengers are looking for some help dealing with Thanos. With expectations that Phase 4 of the MCU will have an outer space focus, this plot thread could turn out to be quite important going forward.