I think Captain America will continue, as will the Avengers, it will just be that the cast will change. Bucky is an obvious choice to become the new Cap and there's always Falcon too. Thor I think is less likely to continue, changing him to a woman is an option but an unlikely one for now. Then I think (based on Age of Ultron) - Iron Patriot could be the new 'Iron Man', with a role reversal for a solo movie (Iron Patriot being the main protagonist with Iron Man just being the sidekick).
Then with whichever of the new stars take off (Ant-Man, Black Panther, Captain Marvel, Doctor Strange) - I full expect Phase 4 to end with a 'New Avengers' movie. Although it's a long way off, there are many stories that could be used for material for more Avengers films and with Inhumans launching at the end of this 'era' I wouldn't be surprised if one big plot point is Avengers Vs. Inhumans (Inhumans are after all the MCU mutants).
It may all seem the distant, uncertain future, but I would be shocked if Marvel don't have a Phase 4, 5 and 6 at the very least in their minds.
I do admit, if they keep a new generation of Avengers going strong for another three phases, the climax of phase 6 would be an amazing opportunity to bring back Iron Man or something, in a typical "old man advises the cast" kind of role, so that we can see a post-Avengers Tony Stark twenty years from now, and to make the whole thing feel like one long epic, rather than a continuing soap opera of superheroes.