Follow on to Casini should be Titan balloon mission
There are some missions around that are just inescapably cool. The DAWN mission to visit Ceres and Vesta is in the "cool" category. As Cassini attempts to unveil some of Titan's mysteries, it becomes apparent that will have more questions than answers when the mission ends. The thick Titan atmosphere creates many difficulties for conventional probing with optics. At the same time it offers a tremendous opportunity. The power requirments to maintain a 400Kg payload buoyant in Titans atmosphere is 100 times less than to do the same on earth. Check out one mission proposal.
titan mission proposal
Montgolfiere Balloons Tech