Here's one for a submarine on Titan - https://www.nasa.gov/content/titan-submarine-exploring-the-d...
There was another Titan boat proposal a couple years ago, but it lost NASA's internal funding battles to Yet Another Mars Lander. http://www.universetoday.com/96924/mars-lander-wins-out-for-...
The biggest issue, however, was the problematic development of the needed power supply[1].
There is another option for these missions, of course. A nominal increase in NASA funding to increase the number of missions we can fly.
[1] https://en.wikipedia.org/wiki/Advanced_Stirling_radioisotope...
TiME was always a longshot proprosal, but I'd personally rather see the lower budget Discovery Program missions doing riskier stuff. And while I love a good Mars mission, Mars gets all the marquee missions anyway. It's kind of a shame to see it eat the resources of the lesser missions too.
over a decade or more, though :) I just wish we could have more.