While we don't know anything about Marvel Phase 4 yet, everyone is almost sure that we're going to get a slate announcement next week at SDCC 2019. What's interesting is, a casting call has possibly hinted at the appearance of one of Marvel's oldest heroes, Namor, the Submariner.
As per The GWW, a Marvel Studios film is set to film in Australia, and the casting call is looking for a "wise, old statesman" who is said to be "the leader of his people." The role is looking for Asian actors aged 40–60.
A second character is said to be "one of the leader's most skilled warriors" and the studio wants a younger Asian woman in the role, aged 30–40.
The site speculates that the casting could either be for Black Panther 2 or Shang-Chi, with the former more likely since the film is aiming to shoot soon, and since they don't need to cast main characters, it would make sense they would only be looking for these two.
While it's not confirmed that it could be the Submariner, some think that Okoye's line about underwater earthquakes in Avengers: Endgame could already be a hint. With Namor running his own secret kingdom, it would be interesting to see him clash with T'Challa in the next movie, maybe even become an ally at some point. I'm sure not every fan would just let it fly that Namor would be a villain. Then again, it would be interesting to see how they differentiate him from DC's Aquaman franchise.
Again, this is all just speculation, but hopefully we get some solid updates next weekend at SDCC 2019.
For now, the only MCU movie with the final schedule is Spider-Man: Far from Home and currently in theaters.