SPOILER WARNING: Marvel and Disney are all in on spoilers now, but I'd rather play it safe than sorry. Big spoilers for Avengers: Endgame to follow!
Avengers: Endgame was indeed the finale that we were all promised. The film closes the door on some pillars of the MCU, including Thanos, who's been the big bad of this whole franchise. With the Mad Titan seemingly gone for good, who else could possibly terrorize the Avengers for the foreseeable future? Well, I don't know about the final boss, but as far as Phase 4 is concerned, Endgame may have paved the road for a classic Avengers bad guy in the form of Kang the Conqueror.
As the millions of people worldwide who have seen Avengers: Endgame know, the entire second act of the film is devoted to time travel. In order to undo the Snap and bring back everyone who was lost, the Avengers take a trip through their history to find the Infinity Stones, bring them to the future and not completely break the laws of time and space in the process.
The time travel rules and implications in the blockbuster have been hotly debated since release. While time travel continues to be one of the most confusing elements of Avengers: Endgame, the mere appearance of time travel could signal that Kang is right around the corner.
The name Kang is likely to be unfamiliar to anyone who doesn't read the comics, but he's one of the more standout Avengers villains. Essentially, he is a warlord from the future who has mastered time travel technology and travels to different timelines in order to conquer them with his superior weaponry. He's basically taking a test with all the answers written on his hand.
Kang's time traveling antics have resulted in him crossing paths with the Avengers on more than one occasion. It usually boils down to Kang trying to manipulate the events of time in his favor, and the Avengers have to stop him. He's a master-level historian and strategist, and his futuristic technology is so advanced that even some of Earth's greatest minds can't fully comprehend it.
Kang has traveled through time so much that there are almost unlimited versions of himself, so he's practically impossible to truly kill. Sometimes these alternate versions have gone on to have their own identities and have been both allies and enemies of the Avengers, such as Kang's future self, Immortus, or his younger self, Nathaniel Richards.
As you probably gathered, time travel is Kang's thing, and we just had a major Marvel movie in which time travel was an integral element. So it's not beyond the realm of reason that all this messing around with space-time will bring in the Avengers villain whose one thing is that he time travels.
Like every movie that deals with timeline jumping, Avengers: Endgame makes sure to explain all the rules of its particular brand of time travel. Nothing you do in the past can affect your future, but if you make a big enough change, it'll cause an alternate reality to branch off. Maybe that alternate reality will be great, but it also has the potential to be dark -- especially if an Infinity Stone is taken off the board.
Captain America makes sure to put the Stones back in their rightful places at the end of the film, but that doesn't mean that the Avengers are off the hook. They made some serious screw-ups in the past that caused alternate timelines. For example, Loki escaped with the Tesseract in 2012, causing a pretty huge deviation. Additionally, 2014 Thanos and his army were killed, which means there's a timeline with no Thanos searching for the Infinity Stones.
Those are just two of the biggest examples we know of, but as Tony Stark says, when you mess with time, time tends to mess back. Maybe the Avengers' actions in the past have caught the attention of Kang, who has now set his sights on Earth.
It's not too hard to see the meddling in the timelines as a good entry point for Kang. Maybe in the MCU, Kang is more of a cosmic force policing time and looking to punish the Avengers for their actions. Or maybe messing with time has opened the door for Kang to do some conquering, and a new team of heroes must assemble to fix their mistakes. Thanks to Spider-Man: Far From Home, we know that there's a multiverse of options for Kang.
There are a few reasons why Kang being the villain of Phase 4 is significant. For starters, he technically debuted in a Fantastic Four comic. It was when he was posing as a pharaoh named Rama-Tut in ancient Egypt. He didn't start going by the name Kang until he showed up in an Avengers comic.
That made the issues of who owns his film rights a little complicated, but it doesn't matter anymore. Disney famously bought Fox, which means the studio can now use all the X-Men and Fantastic Four characters it wants. That means Kang is free to show up down the road.
Because Kang first showed up in a Fantastic Four comic, the villain might be a good way to bring the Fantastic Four into the fold. While we have zero clue how Marvel will incorporate its First Family into the MCU, one theory gaining steam involves the multiverse. Some people think the Fantastic Four will be from an alternate timeline, which could help connect them to Kang.
Also, Kang is a future descendant of Reed Richards, a.k.a. Mr. Fantastic, so that's an interesting angle.
Oh, and Kang has connections to the X-Men, too! When he was a pharaoh, he tried to manipulate a young Apocalypse to be his heir. This guy has his fingers in pretty much every pie.
Kang also has ties to a different superhero team that people expect to show up in the MCU very soon. The Young Avengers are a team of superhero teenagers who have styled themselves like their favorite Avengers. One of these kids is Iron Lad, who also happens to be Nathaniel Richards, a young version of Kang who is trying to escape his destiny as a conquering A-hole.
Having Kang in the mix could be a way to help set up the Young Avengers down the road. However, the MCU's version doesn't necessarily have to use Iron Lad or Nathaniel Richards at all.
Admittedly, Kang is not set up in the way past MCU villains have been. There's no post-credit tease, so the door is open for almost any villain to take charge in Phase 4. However, given that Avengers: Endgame dived into the deep end of time travel, it makes sense that Kang would be the next villain up to bat. Not only is he one of the bigger Avengers villains still left, but he has ties to multiple hero teams and could help set up more movies down the road. Having a time traveling villain could make things more confusing, but it would also be a hell of a lot of fun.