Avengers Phase 4 is underway and Phase 5 is just a few years off, what with 4 being planned to be the shortest phase thus far. Not that that short time frame has allowed us to be privy to what Phase 5 has in store for us, mind, but we can definitely make some educated guesses.
The Fantastic Four will definitely show up, as will the X-Men, and of course it's only a matter of time before we get a new Avengers film.
Marvel is definitely wise for giving it as much time as they currently are before continuing off of Avengers Endgame. But who will actually, ya know, be ON the team when it does happen? After all, besides Mark Ruffalo - who thanks to Endgame fusing him and The Hulk together permanently, can officially play The Hulk for the rest of his life - the original Avengers cast have said their goodbyes for the most part to the MCU. So we need some new blood, and fortunately, Phase 4 is giving us a lot of options to work with on that front.
These are the heroes that I predict will be part of the New Avengers in MCU Phase 5.