Kevin Feige already said the MCU will be a lot different for Phase 4, and it's also been said James Gunn's Marvel Cosmic Universe will be a part of Marvel Phase 4.
Now MCU producer Nate Moore offers a hint as what's to come, though doesn't go into specifics. Nate Moore is a producer on the new Black Panther movie and has been producer on Captain America: Civil War, Captain America: The Winter Soldier, and has been involved with other MCU projects.
Nate Moore tells the UK's Complex Phase 4 will be going with new heroes and at the same time moving forward the current heroes:
So we’re finishing up what we call Phase 3, which will sort of end with the fourth Avengers film in 2019. And then I think for us, we continue that conversation of, what’s next? Who will be the new heroes who can tell great new stories and bring us to new worlds that we haven’t even had a chance to visit yet, while still expanding on the slate of heroes we have and finding the next step for characters people have come to love? We’re looking for the next best story. And then we let the story tell us what we should be making as part of that one united narrative in the Marvel Cinematic Universe.
As Nate Moore mentions using new heroes, he also says the MCU has the fans in mind:
At Marvel, the fans are at the forefront of our minds when we talk about development. What does our audience want and how do we give it to them in a way that’s unexpected? What do they want to see? What can we show them that they don’t know they want to see, but that we think they’ll really respond to? And a big part of that is plunging into new worlds with characters that offer a different flavor than what people are used to. Those are the stories that we want to explore.
While I can't speak for all the MCU fans, I know a Richard Rider Nova movie is wanted by a lot of Marvel Cosmic fans. For the street-level fans, I know Moon Knight is a hot topic.
Who would you like to see a part of the MCU Phase 4?