I think you need to spend an evening with the search button, there have been too many quality threads on the subject to condense them all here.
However, just to blow my own trumpet and give you some ideas, these are two trips i've taken up north (although no further than the cairngorms)
http://www.singletrackworld.com/forum/topic/blair-atholl-to-ft-william-via-aviemore-with-idiot-glen-tilt-night-ride-content
http://www.chargebikes.com/projects/project/files/Ian_Ezzi/
Maybe search for "Kenny Wilson", any thread with his name in it is probably a discussion on highland trails, there was one a while back with some pictures of a bealach above Glen Affric which looked spectacular, but i couldn't find it on search. On that note, his book is a great way to start planning, as it features some spectacular natural trails, all over scotland, which can all be linked together with some crafty OS work.
Edit: Ignore all of that, I just noticed you said you weren't touring.