Ever since Iron Man was released in theaters nearly a decade ago, the Marvel Cinematic Universe has been fulfilling comic book lovers' dreams and shattering countless box office records. The first three phases of the MCU all lead up to 2018's Avengers: Infinity War, and that film's action-packed first trailer proved that the best is still to come.

Marvel Studios president Kevin Feige has stated that the 20 films that will follow Infinity War will be "completely different from anything that’s come before," so fans have been filling the Internet with theories about what might happen in the massive crossover epic and how Infinity War might affect the future of Earth's Mightiest Heroes. Some of these theories have a lot of evidence backing them up and, if they're true, the Avengers as we know them will never be the same.

15 Heimdall secretly possesses the Soul Stone

At the end of Thor: The Dark World, Sif and Volstagg gave the Aether to the Collector because it contained the Reality Stone and it's dangerous to keep multiple Infinity Stones in the same area—Asgard already had the Tesseract and the Space Stone within it locked away in their vaults. Well, several fans seem to think that Asgard is still in possession of two Stones, because Heimdall is the secret wielder of the Soul Stone! Heimdall seems to have the ability to see souls on any of the nine realms, and he was capable of spiritually communicating with Thor when the God of Thunder was stuck on Sakaar in Thor: Ragnarok. Heimdall's eyes are a bright yellow, the same color as the Soul Stone, and in Thor's Age of Ultron vision, Heimdall was blinded and his eyes were pure white instead of yellow. Was that a sign that Thanos will strip Heimdall of his Soul Stone in order to complete the Infinity Gauntlet?

14 The real reason "Hawkeye" isn't in the trailer

We absolutely loved the trailer for Avengers: Infinity War, and since it broke several records on YouTube, we're pretty sure most fans felt the same way. There was one major problem we had with it, though—Hawkeye was nowhere to be seen! Luckily, one popular fan theory might offer a valid explanation for the archer's absence. It's possible that after the chaotic events of Captain America: Civil War, Clint decided to quit being Hawkeye. That's exactly what happened after the comic version of Civil War—Clint joined up with the New Avengers with a new name, Ronin, and a darker, ninja-like costume. Marvel probably didn't want to spoil Clint's new identity in a mere trailer, but pictures of Jeremy Renner on the set of the film make us think this theory definitely has merit—check out how perfectly his boots match Ronin's in the comics!

13 Quicksilver will return

Wielders of the impossibly powerful Infinity Stones can alter all of time, space and reality...so pretty much anything can happen in Infinity War. Several fans believe that the Soul, Reality or Time Stones will be used to bring back Aaron Taylor-Johnson's Quicksilver, the speedster brother of Scarlet Witch who died at the end of Avengers: Age of Ultron. We'd be very surprised if Marvel was able to keep Taylor-Johnson's participation in the film under wraps this long, but Quicksilver is a fan-favorite Avenger and it would make total sense to bring him back now that his father, Magneto, may soon join the MCU (due to Disney's recent deal with Fox). Marvel definitely has the means to make this theory a reality, and the Avengers would surely appreciate his help in their war against Thanos and his Black Order.

12 The Soul Stone is in Wakanda

If Heimdall isn't the owner of the Soul Stone, it has to be in T'Challa's technologically-advanced kingdom. Why else would Thanos and his Black Order wage a massive war against Captain America, Winter Soldier, Hulk, Black Panther and the warriors of Wakanda?! In the comics, Black Panther frequently visits the Necropolis where past rulers were buried so he can communicate with them and learn from their knowledge. It'd make sense for Marvel to put the Soul Stone in this Wakandan City of the Dead, and explain that the Stone's power is what allows T'Challa to speak to previous Black Panthers. The trailer makes it look like Thanos will save this Stone for last, and will fight (and possibly kill) several of our favorite Avengers to get to it.

11 "Infinity War" won't be the end of Thanos' story

Massive spoiler alert! Last summer, Gamora actress Zoë Saldana accidentally stated in an interview that she was getting ready to film "Avengers: Infinity Gauntlet." James Gunn quickly told the press that Zoë misspoke and meant "Infinity War," but with Marvel Studios president Kevin Feige previously stating that the title of the fourth Avengers film contains a massive spoiler...we're pretty sure fan theories about Thanos sticking around after Infinity War are spot-on. The war referenced in the title of next summer's film will just be the Avengers trying to stop Thanos from assembling the complete Infinity Gauntlet, but they will inevitably fail and the actual battle between Earth's Mightiest Heroes and the fully-powered Mad Titan won't happen until Avengers 4. When Thanos had the full Gauntlet in the comics, he was able to wipe out half the life in the universe with a snap of his fingers. Let's just hope that doesn't happen in the film!

10 A Hawkeye/Black Widow movie is coming

It seems like Marvel is all about team-up films right now. Iron Man joined Spidey in Spider-Man: Homecoming, Hulk and Doctor Strange helped out the God of Thunder in Thor: Ragnarok and the second Ant-Man film will also star Wasp! Fans have been demanding a Black Widow film for nearly a decade now and some fans think that she's going to get one in the form of a Black Widow/Hawkeye (or Ronin) team-up. These two characters and the actors who portray them are always best when they're together, and if the Avengers fall apart over the course of the next two films, we could totally see them splitting off and fighting bad guys as a dynamic duo. Maybe if this happens, we can finally find out what really happened in Budapest!

9 Carol Danvers will be Ms. Marvel at first

Captain Marvel will be Marvel's first female-led superhero project, but we think for most of the film, the title of "Captain Marvel" will actually belong to a man and that Carol Danvers will be referred to as "Ms." Marvel. Rumors have been circulating that Mar-Vell, the Kree warrior who was the original Captain Marvel in the comics, will be in the film and act as one of Carol's love interests. He developed cancer and ultimately died in the comics, and he inspired Carol to take on the Captain Marvel name. We don't want to spoil the plot of what's sure to be an amazing movie, but you can probably count on the MCU adapting this concept and making Captain Marvel more of a "coming of age" story than one featuring an already established female Captain Marvel.

8 Steve Rogers will be replaced

Chris Evans' contract with Marvel is about to expire, so the moment we've all been dreading for years is coming. Steve Rogers is going to die in either Avengers: Infinity War or its sequel, and the title of Captain America will have to be passed onto one of his heroic best friends. In the comic versions, both Bucky Barnes and Sam Wilson have adopted the iconic Captain America mantle, and one of them will be chosen to do so in the MCU in the very near future. Since Sebastian Stan's contract states he'll do four more films after the fourth Avengers comes out, he's our primary candidate to take over for Steve after his inevitable death at Thanos' hands. These fan-made Photoshops, however, prove that both characters would look absolutely amazing in red, white and blue, so as soon as we're done crying about Chris Evans leaving, we'll be happy to see either of them as the new Cap.

7 Red Skull never really died

At the end of Captain America: The First Avenger, the Tesseract seemingly opened a portal into space and sent the Red Skull through it. Many viewers believe the evil Hydra leader was killed by the weapon, but we have to side with the fans who think that the Space Stone merely sent the Red Skull to some other planet and that he's been planning his return all these years. Johann Schmidt has been a major Marvel villain for several decades in the comics and Hugo Weaving did a great job with the role, so there's no way the MCU would just write him off so easily. We think Thanos will recruit the Red Skull in his war against the Avengers, and that he and Hydra will be back soon enough. Samuel L. Jackson let it slip in a 2016 interview that Nick Fury has been off "trying to figure out what The Skull is going to do," so a Red Skull return for Phase 4 seems pretty likely.

6 The Masters of Evil will rise

What is left for the Avengers to do once they take down Thanos? We don't think any one bad guy can top a godlike being who has all of the Infinity Stones at his disposal, but a whole team of bad guys...now that could be interesting. If the Red Skull comes back for Phase 4, we could see the rise of a Masters of Evil team up that includes several imprisoned villains from past Marvel films. The Leader, Abomination, Red Skull, Vulture, Loki, Helmut Zemo, Yellowjacket... they're all still alive and surely eager to get revenge on the heroes who defeated them. On their own, they were fairly easily taken down, but together, they'd be a force to be reckoned with. Oh, and we love Vincent D'Onofrio's portrayal of Kingpin in Netflix's Daredevil, so can he please be included in the MCU?!

5 Marvel's TV characters will make it to the big screen

Marvel has spent the past decade trying to claim that their films and TV shows are "all connected," but other than a few MCU characters appearing on Agents of S.H.I.E.L.D., we've yet to see any proof of that. It's about time we saw some S.H.I.E.L.D. actors make cameos in an Avengers film, and The Defenders deserve to fight alongside Earth's Mightiest Heroes. Marvel's Netflix shows will have come to an end by 2019, so fans think at that point, they'll finally make the transition to the big screen. Chris Evans will be done with the MCU and several other original actors from the franchise will be leaving shortly behind him, so we need some more characters and familiar faces (okay, maybe not Finn Jones and his subpar portrayal of Iron Fist) in the films.

4 Shuri will become the new Iron Man

Robert Downey Jr. isn't leaving the Marvel Cinematic Universe any time soon. Iron Man was the film that kicked the whole MCU off 10 years ago, and fans have universally loved Downey Jr. for his perfect portrayal of billionaire playboy philanthropist Tony Stark. That being said... he can't keep being Iron Man forever. He's getting old and even in the comics, Tony Stark eventually gets replaced as the armored Avenger. Some fans think Downey Jr. will merely mentor the Avengers after the events of the fourth Avengers film, while Shuri, the sister of the Black Panther and princess of Wakanda, will basically become the new "Iron Man." She's a highly proficient fighter who seemingly uses technology similar to Tony's repulsor beams in the Black Panther trailer, and could be the MCU's equivalent of Riri Williams (the young black girl who replaced Tony and became Ironheart in the comics).

3 A character we know and love is actually a Skrull

Marvel recently announced that the Skrull, a shape-shifting race of diabolical aliens, will act as the primary antagonists in Captain Marvel. The film takes place in the '90s, years before Tony Stark dons the Iron Man armor and starts the MCU, so it's possible that a few Skrull spies survived the events of Captain Marvel and have been posing as Avengers or their allies all this time! That theory sounds a lot like the Secret Invasion comic book storyline, where Earth was plagued with countless Skrull posing as Earth's Mightiest Heroes. We doubt any of the major Avengers who've had their own solo films are secretly Skrull, but it's definitely possible that someone like Nick Fury has been a Skrull this whole time. S.H.I.E.L.D. was infiltrated by Hydra easily enough, so it's definitely possible a few Skrull snuck their way into the organization and have been manipulating the Avengers behind the scenes.

2 Namor will be enter the MCU

Iron Man 2 was filled with subtle Easter eggs that referenced Namor, Marvel's king of Atlantis. Namor's corporation Oracle was mentioned at the start of the film, and his private headquarters located directly off the Eastern coast of North America was shown on a holographic map in one of its final scenes. These references understandably caused fans to wonder if the Submariner will actually enter the Marvel Cinematic Universe, but for the first few years of the MCU, Namor's film rights were tied up due to old contracts with Legendary Pictures and Universal. Now, Marvel has the full rights to the character, so he can enter the film universe any time the studio wants to include him! Reel News Hawaii tweeted about the possibility of both an Aquaman and a Namor movie filming in the area, and while no further details have come out to suggest that was anything more than a rumor, it gave fans hope that Marvel is working hard to bring him to the big screen. They'll need him if they want to do a comic-accurate Avengers vs. X-Men film, and that's actually a possibility now thanks to Disney's deal with Fox!

1 The Illuminati are coming

In the comics, an elite group of Marvel's most powerful and intelligent superheroes joined together with the purpose of working behind the scenes and eliminating major threats to Earth before they actually happen. This team-up, called The Illuminati, originally consisted of Mister Fantastic, Iron Man, Professor X, Doctor Strange, Black Bolt and Namor, and eventually included Black Panther as well. Marvel finally has the rights to every one of those characters, and already has four of them in their shows and films. With Thanos likely killing a lot of innocent people over the course of the next two Avengers films, Earth could definitely use this superteam's help against future cosmic threats. Expect them to appear and align sometime in the next MCU phase or two.