In an interesting interview reported by Norris and Unnikrishnan in, NASA Studies Scaled-Up Falcon, Merlin SpaceX CEO Elon Musk makes the case for NASA building a super-heavy lifter employing an enhanced Merlin engine that would be rated at 1.7 million lb. thrust. Since the Merlin is throttle-able to 60%, this engine could be also used as a replacement in the Falcon 9 and Atlas V. Musk also indicates that the super-heavy lifter would be overseen by NASA with integration managed out of Marshall Space Flight Center and construction done at Kennedy Space Center.
The AvWeek article is a good read and shows the out-of-the-box thinking that has helped SpaceX stand-out.