Both the US and Aussie Air Forces considered upgrading the F-111 to use the GE F110 engine. For the US it was a no brainer option, but the big budget axe that came after the wall fell ensured that no F-111 got the time of day for just about any upgrade, since they were planned to be phased out by the technologically superior, but far inferior airframe of the F-15E anyways.
http://www.secretprojects.co.uk/forum/index.php?topic=803.0
The Australians also considered some vast improvements to the 'Vark fleet as they also knew the F-111 airframe was far superior to any other new build "long range" strike platforms, but would have required such massive structural overhauls to the airframe to lengthen their service life, that they might as well have been rebuilds. A real shame, since new builds would have solved that issue, but the production tooling was loooong gone by then.
http://www.ausairpower.net/TE-F-111-Upgrades-1998.html