Robert Downey, Jr. set to join the cast of Captain America 3, which will have a Civil War storyline with Tony Stark/Iron Man and Captain America leading different factions of superheroes divided over the Superhero Registration Act.  Captain America 3 will start production next spring for a May, 2016 debut.
 
Variety reports that RDJ has only a third Avengers movie on his current deal with Marvel, so the new deal for Captain America 3 could well include a fourth Iron Man film, which Downey appeared to be lobbying for  when he was doing publicity for The Judge (see "Robert Downey, Jr. Confirms 'Iron Man 4'").
 
Being the “heavy” with good intentions in Captain America 3 would give Downey a chance to show another, not very flattering side of the Tony Stark character.  As if by some strange coincidence Marvel today began teasing a reboot of the Civil War event (see "Marvel Teases a 2015 'Civil War' Event").