According to Chris Evans himself (via Collider), he is open to return as Captain America as long as Marvel Studios wants him. 

After going back-and-forth on whether he would be extending his contract after it’s up post- AVENGERS: INFINITY WAR, it looks like Chris Evans is firmly entrenched in the Marvel camp. This is tremendously exciting news for everyone who has found his Steve Rogers to be the heart and soul of the Marvel Cinematic Universe.

This doesn’t mean that we will get more Evans after the two-part Avengers film, but it does leave the door open if Cap survives the proceedings. I certainly hope that we get a few more adventures with the character before he hangs up the shield, but at the same time understand that these roles have an expiration date on them. Whether we get two more movies with Chris Evans or six, we should be happy we got any at all. His Captain America has been an unexpected highlight of these movies and a moral compass for the people within the MCU and those of us outside it.