These are all great ideas, and they highlight one important aspect of music, harmony. Since the Dead were much, much deeper
harmonically than most anyone else considered a 'rock' band, this aspect tends to get the most attention. But even if all you know is a basic pentatonic scale, there's a ton you can do to build the tension in your improvisations.
Dynamics: Build toward a big loud, crescendo, cut back to a whisper, etc, etc. How many slow blues have you heard that get pin-drop quiet in the middle and come back in screaming? Any good symphony recording of the classics is a seminar in dynamics.
Rhythmic. Start out floaty, build to faster, denser rhythms, or resolve a flurry of notes to a long sustained one
Sequences :play something, then 'answer' it higher or lower with something similar. Keep it going for as long as musical taste or length of fretboard allow.....
Timbre, change up the tone w/ your hands(pick closer to bridge, snap strings, etc) or feet(dist+,mutron, etc)
Be mindful of things you listen to that successfully build tension, and keep track of what devices they use to pull it off.
Best of luck in your improvisations,