One thing to mention though. While many shows were shot on film, many in the late 80s and 90s were edited on video (Star Trek:TNG, DS9, and Voyager being examples) and any special effects work was done in the video realm.
So, in some ways, re mastering TNG for HD could be quite a big harder than TOS (beyond the increase in number of episodes.)
Now, they did in fact go back to the original film elements and re-edit TOS for the restorations, but the replacement FX were only designed to mimic the model work at the time and therefore simplistic. There was also very little live footage compositing that they had to worry about (and the stuff that was there was VERY simple.)
With TNG, they would have to go back to the original film elements, re-edit them, then redo ALL the post special effects work (at a level of quality that would match a show today since the originals weren't too bad) which would be much more extensive than TOS. I really don't have much hope in this happening.