On the other Hand Aquaman has the bigger competion, and Marvel can't do Christmas releases, since that date is reserved for other Disney Movies.
Avengers: Endgame is basically the conclusion everything the MCU has built up to untill this point. I don't think that they will be able to make a movie that will be as big anytime soon.
Giving the mantle to another character is not an easy task as we have allready seen in the comics.
- Spiderman belongs to Sony, I'm not sure in what capacity they can still use him post Avengers: Endgame for shared movies.
- Daredevil, Punisher, Luke Cage and Iron Fist were already used up for the Netflix series, I doubt that they make the jump to the big screen
- with X-men, Deadpool and Wolverine you have still quite popular and very recent incarnations, and integrating a big property like the X-Men in the MCU will probably a big challenge anyway (from a world building perspective)
- what will happen with Guardians of the Galaxy is also unclear do to the James Gunn Situation