Strange isn't really the character to use as a group leader. In the comics, he's been in groups a couple times, but usually when there's a major magical crisis or in an advisory role. Otherwise, he's pretty much a lone wolf type.
As for the original Avengers - I wouldn't be surprised if they do decide to move on from them. In the comics they've made big deals about replacing the original Captain America, Thor, Iron Man, Hulk, and Spider-Man with new versions. It would make sense if they decided to do something similar in the MCU.
My bet for group leader in Phase 4 would be Captain Marvel.
Seeing as Whedon is apparently in talks to do a Batgirl film, I don't think he'll be making any more MCU movies.