Question about the Avengers movies?

As far as I'm aware, Infinity War will round off the stories of the current movie phases, but afterwards Marvel plans to look at new characters in new story arcs. For example there will be one Captain Marvel movie before Infinity War, but afterwards they could easily make more.

Its unlikely they'll make more with their core cast (Iron man, Thor, Cap) but they'll instead focus on the others, and give new characters a go.

Marvel hasn't said anything beyond Infinity War, which is the cap on the end of Phase 3 of their movie plans (which was all sketched out as soon as the first Iron Man movie was a huge success). Chris Evans (the actor for Captain America) has said he's pretty much done with acting as soon as his contract with Marvel is up, instead wanting to go into directing. Robert Downey Jr. has also suggested that he's not inclined to keep going as Tony Stark, because he's pushing 50 (which makes the physical aspects of a superhero movie more strenuous) as well as that, as Iron Man, he actually has to do more acting work, since after every one else is done, he has to then go into a booth and shoot all the shots from inside the helmet (which he compares to voice acting, meaning he has to focus on a different set of skills that he doesn't normally use much of). Scarlett Johannsen (Black Widow) has commented that she regrets signing on for 8 (!) movies in this first three phases, as that just eats up almost all of her schedule, and seemed to be annoyed at the lack of a Black Widow movie. The others haven't really hinted much on their futures either way. It will also be interesting to see, as Joss Whedon steps back a bit (he's confirmed to not be directing Infinity War), what will happen to the quality of the films (current rumor is that the writer/directors of Captain America: Winter Soldier, will be taking over directing Infinity War).

As far as replacing or recasting the characters, the large number of new characters getting their own movies in Phase 3 suggests that at the time they planned this out, Marvel wasn't sure what they were going to do. They do have options, not just of recasting the actor playing the same character, but also changing which character is which superhero. After the Marvel Civil War storyarc in the comics, Steve Rogers gets shot in the head (presumed dead, but actually alive and kept in stasis by SHIELD until they could figure out how to heal him), and the shield of Captain America changed hands between a few other heroes, including the Punisher and Bucky Barnes. Cap has since come back, taken back the shield, then had the Super Soldier serum removed from his body which (inexplicably) physically aged him to what he should be, and he gave the title of Captain America to Falcon. Also, for a while, War Machine was wearing the Iron Man suit in the comics, because Stark was deep in a bout of alcoholism.

My assumption would be that, moving forward, they are going to want a more diverse team moving forward, and Captain Marvel (the Carol Danvers version) and Black Panther seem to fit the bill, since both are getting their own solo movies in Phase 3. (It would also be interesting to see the interaction with Rogue and Carol, if Marvel ever gets the movie rights back for X-Men.) Joss loves to kill off main characters, and both Iron Man and Captain America seem likely. Captain America has a ready-made, comics-true replacement already established in the movies with Falcon, and honestly, now that they've established in the movies that the arc reactor can just be built into the suit, anyone could be Iron Man, should they kill him off and decide later to bring the superhero back.