Construct solos that follow the melody line. It can be as simple as restating the melody or as complex as you'd like to make it - however, does it "fit" in the song ? The solo is part of the overall song - the song isn't created to showcase the solo ( exceptions noted ) so the solo should fit into the mood / message of the song. In the Dead's case, as a jam band, expanded solos would usually take the music beyond the melody line, but the melody line in most cases was used to tie the entire jam together.

Why limit yourself to 3 note per string scales ? Creatively combine the various scale positions on the fretboard to open up greater possibilities. Learn to incorporate double stops ( 2 notes played together ), bends, slides and other techniques to enhance your solos. I understand the concern for limiting horizontal hand movement, but if you want to wank away on 3 notes per string scales at 200bpm, be my guest. Not very musical (to me, other may disagree).