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.