Q:

Any proposed missions to explore the black liquid on Titan? Technical challenges?

I was very interested to know what the black lake on Titan would look like. Are there any plans or proposals for a future mission to land on and/or in one of the lakes of black liquid on Titan, or even to land nearby and approach it?
Would there be any unique technical challenges to this that have not been encountered and solved for previous lander missions?

A:

It's a late answer but I was surprised nobody cited this:
Titan Mare Explorer (TiME)

The TiME lander was a part of cooperative NASA-ESA mission TSSM. The mission was not considered as highest priority by Planetary Decadal Survey 2013 so it's not being implemented now. Also it was studied as standalone Discovery class mission.
ASRG generator was planned as compact energy source but its development didn't end well enough.
Goals and instruments:

Goals:
• Understand Titanʼs methane cycle through study of a Titan sea.
• Investigate Titanʼs history & explore the limits of life
Instruments:
• Meteorology & physical properties (MP3)
• Mass Spec for Lake Chemistry (NMS)
• Descent and Surface Imaging Cameras

