It seems the questions of how Marvel deal with ageing actors, ending contracts and the need to diversify the slate of characters are ones they're consciously seeking to address. The answer seemingly lies in the conceptualisation of the first three Phases as a trilogy (or a saga in more conventional terms), and the Phases beyond that as a whole new world. The impetus - for Kevin Feige at least - is on difference:
"I think there will be a finality to moments of Phase Three, as well as new beginnings, that will mark a different, a very different, a distinctively different, chapter in what will someday be a complete first saga made up of three phases."
So what exactly does that mean?
We can already expect Guardians Of The Galaxy 3 to be coming, there's bound to be a Doctor Strange sequel (though quite what they'll do with it remains intriguing), and there's a distinct chance we'll see more Ant-Man, more Black Panther, and more Captain Marvel, by virtue of them being the new guys. Spider-Man sequels are likewise inevitable, and it wouldn't be too much of a stretch to imagine Inhumans being shunted out of Phase 3 too.
But what of the wildcards? What of the new or braver projects that Marvel could tackle that really would make Phase 4 different from what has come before?