We're staring down the barrel of the end of Phase 3 of the Marvel Cinematic Universe, but, at this point, we don't know much at all about what the studio has planned for Phase 4. Apparently, we're not going to know for some time. According to Marvel chief Kevin Feige, he'd like to keep as many details of Phase 4 as quiet as possible until the current phase wraps up with Avengers 4. This means it could be another two years before we learn what's next for the MCU. According to Feige...
I hope [the news comes] after that. I think there will be certain elements that start to come about, not the least of which is the already announced release date of the next Spider-Man film. But other than that we're going to keep it very close to the vest because this is really about focusing on the unprecedented conclusion to a 22-movie overarching narrative.
At this point, we only have two things confirmed following the end of Phase 3 and Avengers 4. The sequel to Spider-Man: Homecoming will be released two months after Avengers 4 in July, and Guardians of the Galaxy Vol. 3 will happen, we don't even have any real idea when. Beyond that, we're in the dark, and based on what Kevin Feige says to i09, we may very well stay that way. It's possible that it could work too. If Guardians of the Galaxy Vol. 3 ends up being the next MCU film released after the Spider-Man sequel, in say, May of 2020, then it's possible that nothing else will need to be in production before Avengers 4 hits theaters. Once cameras start rolling at Pinewood Studios, or wherever Marvel starts filming, hiding what they're doing will be difficult, prior to that, however, while rumors will certainly still happen, they should be able to keep what they're doing mostly quiet.
And there's a good reason for Marvel to try and keep things under wraps. In much the same way that Kevin Feige has told us that the reason we don't know the title of Avengers 4 is that it's a spoiler, the same is likely true of the entirety of Phase 4. While Marvel has previously made a big deal of announcing their upcoming film slate, the fourth Avengers movie is set to be the end of a lot of storylines, and learning what films are planned for the next several years would give us all a head start on what the studio has planned. Who will survive Phase 3 could be spoiled by movie titles alone. An announcement of a movie based on an entirely new character would give us more information than Marvel probably wants to at this point.
In order to keep the Marvel machine rolling, they'll need to cast actors and hire directors for films that have not yet been announced, so keeping all their secrets might not be possible. It will be interesting to see just when details emerge, officially or otherwise, on what Marvel has planned for Phase 4.