The basic idea for where I expect marvel to take phase 4 is the idea of many storylines following many teams, like the comics did. So far all of marvel's movies have been interconnected, which is freaking amazing, but all follow one major storyline, many small events leading to infinity war. One of my friends recently, and blasphemously, said that marvel doesn't neccesarily have to do aanything after infinity war. He believes they will just start over and revert back to the original super hero movie blueprint. To this I yell impossible! Marvel revolutionized how superhero movies are made, and should stick with this, well forever. This faces scary continuity issues.
Issues for continuation
Actors grow old. Actors need new contracts to work. How do you keep a storyline going for decades? You have a few options. My favorite is having different people becoming the hero. Bucky will become Captain America, he just will. Miles Morales, at least should, eventually become Spiderman. This does not need to follow comic books at all. I am a huge fan of movies being creative with their characters. You can also kill off or retire heroes. Hawkeye will eventually, if we didn't just see him do so, retire from being a hero. This makes room for new characters to be introduced and explored.
Possibilities for phase four and beyond
Marvel with its vastly successful universe now has options for what to do next. My favorite idea is follow many characters and teams across many storylines. I mean major plots. Introduce dozens of characters, and just run with them. Match them up with other heroes, explore many relationships, interact with many villains, and keep each events intertwining with others from the MCU. Make each movie almost like the equivalent of a few comics that contain their own mini plot line. This can lead to marvel with many teams. Eventually marvel will recollect all of its properties. How about a spidey fantastic four mash up. Or maybe a winter soldier X-Men combination. What if all the active teams, plus a few freelancing mercenaries team up in one over the top conclusion? The possibilities are endless. The greatest fear is that with a much faster developing line then the comics they will run out of storylines and it will become repetitive or run out of characters to bring back from the dead. This leads to a possible phase 6 or 7 idea. Movies create their own heroes and plotlines, and dare I say- replace comic books entirely. Let all of the creativity go to the more exciting and viewed medium- movies.
With a bit of development marvel can have many progressive story lines and teams on their movies, and follow a similar pattern as te comics until they wholly replace them. and seriously, who doesn't want Wolverine training a team of young avengers, who some of which form their own team leading to the creation of an entirely new team. Maybe even deadpool and Spiderman create their own parallel avengers team with daredevil and miss marvel. Like seriously, they can do anything.