The beginning of the end is near. The next movie in the MCU, Avengers: Infinity War is less than a month away. That will just leave Ant-Man and the Wasp, Captain Marvel (a prequel to existing films rather than a continuation), and then an as yet untitled Avengers film, with it Phase Three will be over. And with the end of Phase Three we will potentially see the end of some of the characters. It has been reported that the following actors intend to hang-up their super hero costumes next year: Chris Evans (Steve Rogers aka Captain America), Robert Downey Jr. (Tony Stark aka Iron Man), Chris Hemsworth (Thor). 
This will leave just Tom Holland (Peter Parker aka Spider-Man), and Chadwick Boseman (T’Challa aka Black Panther) as the only remaining Avengers deemed significant enough to have their own films. They will be joined by any surviving cast. They can’t simply recast, this will be conspicuous at best, disastrous at worst. There is another answer within the existing cast: Natalie Portman (Jane Foster), Sebastian Stan (Bucky Barnes aka The Winter Soldier aka White Wolf), Don Cheadle (Lieutenant James Rhodes aka War Machine).
In the comic books on which the MCU is based, these characters have all taken on the part of other heroes: Bucky Barnes – Captain America, James Rhodes – Iron Man, Jane Foster – Thor. 
There have been many other incarnations of the comic books where existing characters have taken on the mantle of other heroes, they include Sam Wilson aka Falcon as Captain America and Natasha Romanoff aka Black Widow as Thor. 
You may remember the scene in Avengers: Age of Ultron when the Avengers take it in turns to try and lift Mjolnir, all except Black Widow, should this tell us something?
Having said all this, they could just introduce some new characters!
Fandango Groovers Movie Blog 