Here's a paper (abstract and preview only) showing how Cassini could be redirected, via Titan flybys, to Jupiter, Uranus, or Neptune, or first to Jupiter and onward via Jupiter to Uranus, Nepture, and a Centaur, and then out of the system ala Voyager. But when this was discussed on unmannedspacefligt.com, the reaction was "WHAT? There is so much more science to do at Saturn, why would you ever leave?!" The people who are operating Cassini are pretty focused on Saturn, and somehow I think having a second probe at Saturn wouldn't lead them to think that Cassini shouldn't stay at Saturn. A bird in the hand.... ....on the other hand, it looks like the options for Cassini are still quite varied, and while some of them involve a death plunge like Galileo, other options include a very long term low-resource-usage wide orbit of Saturn doing seasonal observations of Saturn Titan, and Encelasdus, which means Cassini might yet play a key role in the glorious eight planets at once observation campaign.
pdf.aiaa.org/preview/CDReadyMAST08_1856/PV2008_6753.pdf