Once phase 3 of the MCU ends with Avengers 4 we will have Phase 4 to look forward to. We have been told that the 4th Avengers movie will change everything in the MCU, so they may not even continue using the phase format.
The first movie in what is currently being called Phase 4 is going to be the sequel to Spider-Man: Homecoming. This film is going to be released July 5 of 2019. The third Guardians of the Galaxy movie is also going to be in Phase 4, but the time has not been announced yet. There are also three untitled movies that will be released throughout 2020.
I think that one of these movies is going to be a Doctor Strange sequel. I also think one of these is going to be the long awaited sequel to the Incredible Hulk movie. The third movie is going to be a new franchise since Marvel likes to have two sequels and one new franchise each year.