I wish we had built a bunch of Cassinis- one for each gas giant. There's so much about the outermost planets we don't know. Voyager missed the weather changes as Uranus and Neptune changed seasons- a flyby just isn't enough!
To be fair, I get the impression that no matter where we look in our solar system, we discover heaps of unexpected stuff, and it's all fascinating. But compared to Jupiter and Saturn, we still know fairly little about Uranus and Neptune. :-(
Sadly, I think it's rather unlikely this will change anytime soon, but a person can dream...