The MD-11 was -seriously- considered as an additional tanker/airlift model for the USAF but the idea was dropped. A designator number given as "KC-10B" for this model was brought up several times. It featured the strengthened floor and cargo handling of the KC-10A with a much more massive fuel offload/carry capability. At the time it was being considered, the C-17 was in a state of doubt and the fleet in service then was just entering into a period of strain.
Air Mobility Command, (then Military Airlift Command) was looking at several "off the shelf" airlifters like this one and a modified 747-400F model. (Of the 747-400F proposal, 12 models were being considered, 6 each at Travis AFB and Dover AFB respectively.) The primary mission for these "off the shelf models" were to handle channel AMC flights moving pre-planned cargo movement like military household goods, maintenance parts and so forth to overseas US bases. Post-Gulf War there was a -very serious- airlift shortfall with the C-141B operating under restrictions due to severe wing cracking outboard of the #1 and #4 engine pylons, which restricted altitude, payload, range and speed.
Add to that a remarkable uptick in overseas deployments after the Gulf and the result was pretty dire. I wont forget it. It worked out by contracting movement of channel frieght using air freight carriers like Southern Air, Konnie Kalitta, International CharterXpress, etc. This bought some valuable time and flexibility.
The C-141B fleet was fixed, overseas deployments eased -slightly-, the C-17 enters service at KCHS. Things started to settle down. So the idea to buy "off the shelf" airliners like the 744/MD-11 was then put to bed. I dont expect to see an MD-11 wear AMC grey, but it almost happened.
As for engine upgrades, it's very possible the C5 -could- receive a CF680 engine upgrade. Depends on weight and of course -cost- (G) The B-52 has also been the subject of engine upgrade studies to include PW,GE and even RR powerplants.