WARNING! This article contains spoilers for Avengers: Infinity War. Do yourself a favor and see it first.
In 2012, six Marvel heroes came together to form a super team in The Avengers. While the Marvel Cinematic Universe has expanded exponentially since then, those heroes -- Hawkeye, Iron Man, Black Widow, Thor, Hulk, and Captain America -- would remain the core group of Avengers until the events of Avengers: Age of Ultron and Captain America: Civil War split them up. To this day they are still considered to be the main Avengers team and the heart of the MCU, but that won't be the case for much longer.
Judging by the end of Avengers: Infinity War and who's still alive, that core team from the first film will be the ones burdened with finding a way to defeat Thanos (Josh Brolin) and reverse his genocidal actions. With Phase 3 coming to an end, along with contracts, and with heroes expected to die or hang it up in Avengers 4, that should make that climactic movie something of a swan song for the OG Avengers team. Disney head honcho Bob Iger hasn't ruled out future Avengers films, and money dictates that they are likely to happen. The only question is, who'll be on the team?
Nobody knows for sure what Marvel will do at this point, but here is some slightly educated guesswork and wishful thinking about which Marvel heroes will probably be Avengers after Avengers 4. Just because someone isn't on this list doesn't mean I think they won't be in Phase 4, just that they might not be a permanent fixture on the core team and will perhaps come in and out. With so many heroes in the MCU and more on the way, the team could get very large. So, to prevent it from getting too unwieldy, I'll keep it to eight members, two more than the original team.
This one is pretty much a no brainer. Captain Marvel was brilliantly set up in the post-credits scene in Infinity War to make a serious splash in the MCU. Brie Larson's Carol Danvers isn't just going to show up in Captain Marvel and Avengers 4 and then peace out to wherever she's been hiding for all these years. Captain Marvel should play a huge role in Phase 4 and I imagine that she may even be the leader of the Avengers going forward as she was for the Mighty Avengers in the comics. Captain Marvel is a born leader with a military background, who always fights for what she believes in, no matter what, making her a great successor to Cap's role.
I debated whether or not to include Black Panther on this list. Obviously he will be in Phase 4 and Marvel will probably include him in as many MCU entries as possible given the success of his debut. But I'm not sure that a king has time to be part of a superhero team on a permanent basis. However, now I'm thinking that post-Avengers 4, after the heroes save half the planet, the Sokovia Accords will be ripped up and heroes will be accepted again. With Wakanda opening itself up to the world, T'Challa will join the team as something of an ambassador, representing the country on the world stage as a part of the Avengers.
Chris Hemsworth's contract is up after Avengers 4 but of the big three (Iron Man, Thor, Cap), I think the God of Thunder is the most likely to stick around into Phase 4. Now that Marvel has finally got the character right and delivered a great Thor movie with Thor: Ragnarok, Chris Hemsworth seemed to be really enjoying this reinvention of the character and seems excited about continuing with him. Thor could maintain the bridge to the MCU's cosmic elements, and who doesn't want to see him yucking it up with new characters like he did with Rocket in Infinity War. Asgard is gone so he can call Earth his home now. Thor would also provide the connective tissue between the Avengers' past and their future.
Doctor Strange is a natural fit to properly join the Avengers after Avengers 4. He covers a huge blind spot by handling the magical side of things and Benedict Cumberbatch has only had one solo outing with the character thus far. With the longevity of Marvel contracts, I imagine he is due at least another film or two. Doctor Strange was a standout in Infinity War and many walked out of that film liking the Master of the Mystic Arts way better than before. He seems to work better with other heroes to play off of. Plus Doctor Strange fulfills the team's arrogant jerk quota that we assume Tony Stark will be vacating.
I'd really love to see Elizabeth Olsen's Scarlet Witch continue on into Phase 4 and have a big role there. Wanda Maximoff plays such a major part in the comics and is a longtime Avenger, and her continued presence as a part of the team would open up exciting event stories that the MCU can adapt like House of M. She really stepped up in Infinity War and showed how much of a vital team member she has become. We've only begun to get to know Scarlet Witch and the extent of her powers. She could also be a potentially chaotic presence on the team, as the Hulk was once, which makes for some great drama among the members.
This team can't be all people with god-like powers and while he'll probably never get his own solo movie (or his comic book suit), Anthony Mackie's Sam Wilson is a great supporting player who is entertaining when he's on screen and is always good for some comic relief. The actor seems to really enjoy being in the movies too. Also, with Chris Evans' Captain America expected to bite the dust, it has been presumed that either Sam Wilson or Bucky Barnes will take up the mantle. That could go either way, or neither, but Bucky is more of a loner, so new Captain America or not, I think Falcon would provide something different to the new Avengers team.
Black Widow has been in the MCU since Iron Man 2, so it wouldn't be surprising if Scarlett Johansson were ready to call it quits. However, the recent talk about Black Widow finally, at long last possibly getting her much-requested solo movie, makes me think that she'll stick around for a while longer. The Black Widow movie may be a prequel but I think Marvel may throw some money at her to keep her around for team-up films as well. The Avengers team needs a more grounded member, versed in spycraft and Natasha Romanoff fits the bill. Also, Tessa Thompson has hinted that Phase 4 will be focused on the female heroes and having three women on the team, including this OG fits with that.
We don't know much about Phase 4 yet, but one of the things we do know is that Spider-Man: Homecoming 2 will be the first movie out of the gate after Avengers 4. So obviously Tom Holland's Peter Parker will be a presence moving forward. With Spider-Man proving himself a great team member and capable fighter against Thanos, he may be a full-fledged Avenger post-Avengers 4. I have no doubt Marvel wants to use the character to the fullest given his popularity, the only trick with Spider-Man is the Sony-Marvel deal. That might result in Spidey being kept somewhat separate from the rest and only coming around to help here and there, just in case the deal isn't re-upped.