Marvel Studios’ President Kevin Feige has a plan for Marvel movies that goes well beyond those titles that have already been announced.  Fiege told WIRED "I could arguably say what we are planning for the year 2021.  Will that happen?  I don’t know.  But what we planned for 2015 in 2006 is happening."  While Feige wasn’t ready to share Marvel’s slate of upcoming films beyond what has already been announced, there is no reason to doubt that this architect of the Marvel movie onslaught does indeed have his movie “ducks” lined up in a row for years to come.
As WIRED points out in its lengthy profile of Feige, he was behind the teasers at the end of the Marvel Studios films that are not just obvious sequel references, but entrancing clues to future Marvel films that may be only tangentially related to the movie that just screened, but which definitely whet the appetites of fans and stimulate plenty of water cooler conversations.
Joe Russo, one of the brothers directing Captain America: Winter Soldier, called Feige "an auteur producer," which has to stand as a major tribute to Feige’s behind-the-scenes work in planning and facilitating Marvel’s diverse slate of films.  The Russos were appreciative of Feige’s flexible approach that allowed them to make Winter Soldier, the sequel to the World War II military action-themed Captain America: The First Avenger, into a 1970s-style political thriller--and Feige has also given the OK for Edgar Wright’s “caper movie” take on Ant-Man, which is currently slated to debut in 2015. 

But it appears that the one movie that could have the biggest effect on what Marvel Studios actually releases in 2021 is the next non-sequel Marvel film Guardians of the Galaxy.  A space opera saga based on a little known Marvel property, Guardians will be a sort of litmus test to see if Marvel Studios can reach behind the first (and second) ranks of Marvel characters and still create a successful movie franchise.  Feige again appeared very flexible and "outside the box" in his approach to Guardians, hiring James Gunn, who managed to inject large amounts of humor into his previous genre films (like the horror film Slither), to direct Guardians.  If Guardians is a success, we could be seeing a Guardians/Avengers team-up in a battle to the death with Thanos.  If it’s a failure, according to WIRED, Marvel Studio’s may go in a more psychedelic and conventional fantasy direction with Doctor Strange, if the Doc’s 2015 debut effort is a success (see "Doctor Strange Movie Confirmed").

Whatever happens, it appears that Feige will have a plan for it, a plan that will connect and reinforce a whole slate of Marvel movies.  Of all the advantages that the current massive slate of Marvel movies has when compared with DC and Warner Bros.’ offerings, the interconnectedness of the Marvel films, the transposition of large parts of the Marvel universe from the comics page to the big screen just might be the biggest.