Currently, with Iron Man 3 in production and preparing for its May 2013 release, Downey is contractually working on his last Marvel Studios movie. That is unless they re-sign him to appear in The Avengers 2, which would be a no-brainer.
An assumption would have to be made that Iron Man 4 might not be in the cards for Phase Three—because of Downey's status being up in the air—and the Ant-Man movie could replace it instead. (Just our guess so do not quote this as being fact please.)
If Marvel re-signs Downey on for The Avengers 2, Marvel could re-cast a younger look-a-like for Downey if he is indeed finished playing the Armored Avengers on the big screen after The Avengers sequel. Maybe they would bring on actor Dominic Cooper, who played the young Howard Stark in Captain America: The First Avenger. (Just another guess on our part.)
In the end, the future of Downey and Marvel Studios is a mystery right now and we will probably have to wait and see how things play out for him after Iron Man 3 wraps and is released in May. It would be a sad day to see Downey leave the role(s) his has owned on the big screen but maybe there is a slim chance he wants to play the character(s) as long as he can—just as Wolverine actor Hugh Jackman has said about retaining his role, a few weeks back.
We can only cross our fingers and hope Downey can show the same appreciation as Jackman since those same roles as Stark and Iron Man literally set his career back on the path of being a Hollywood superstar.
More stories on Iron Man:
Iron Man 3 Production is Way Behind Schedule, Per Report
Iron Man 3 News: The Hulk and Dr. Bruce Banner Will Not Have a Cameo
SDCC 2012: Iron Man 3 Footage From Comic Con Detailed Along With New Armor Specifics - SPOILER ALERT