It has been no secret that Marvel wants RDJ back in their movie franchises. Downey has been up and down as to whether or not he would be looking to return as Ironman. Going as far as to say that it would be better to retire on his own terms before he is basically too old to play the role.
It seems as if RDJ knows that he probably has a couple more iconic movie roles in him. He does not want to be remembered as the failed Doctor Dolittle!