Grace Wehniainen of Bustle explains that deciphering the Avengers 5 line-up is difficult because the definition of "Avenger" is not as clear-cut in the MCU. Kevin Feige added to the confusion with a statement indicating the question of what it means to be an Avenger, plus the matter of who's financing the team and leading it, will be a question lingering in the background of upcoming MCU projects.
While Marvel Comics suggests that Avengers are typically recruited, the MCU seems to present a grey area. The original six in 2012's The Avengers may have been assembled by recruitment, but there have also been several times throughout the Infinity Saga where the "Avenger" label could be applied to any character fighting on the side of good. In the case of Shang-Chi, Marvel Studios had already hinted at his Avengers future by marketing him as "Marvel's New Avenger" even before the movie came out. But there was also a potential Avengers 5 clue in the Eternals trailer, in which Ikaris jokingly names himself the next leader of the team. Wehniainen says that the latter example implies that any of the "super-powered 'good guys'" can be a part of the team if they want.
With that in mind, it can be assumed that any superheroes who have fought under the Avengers mantle could potentially be part of the new line-up. Wehniainen starts with those who fought most closely alongside the original team before and after the events of Civil War: Hulk, Thor, War Machine, Hawkeye, Captain America (Sam Wilson), The Winter Soldier, Spider-Man, Ant-Man, and Scarlet Witch. She then adds a few notable names who assisted the Avengers against Thanos in Endgame: Doctor Strange, Wong, Valkyrie, Shuri, Okoye, Captain Marvel, The Wasp, and the Guardians of the Galaxy.
And those are just the existing superheroes. There are also a bunch of new names to consider, from solos like Moon Knight and She-Hulk to new teams like the Eternals and the highly-anticipated Fantastic Four. Additionally, there's likely going to be some passing of torches between old and new characters (Natasha Romanoff to Yelena Belova, Hawkeye to Kate Bishop, etc.), so that could affect things as well.
Of course, an Avengers 5 film is not likely to come any time soon, as it is way too early in Phase 4 to even know who or what the major threat will be. Feige himself recently said that there needs to be a reasonable amount of time from Endgame to build a new saga before they start bringing everyone together.
The Avengers films are now available on Disney Plus.