Mono instrumental music can be rather impressive. Look at Jimmy Page's sol
rk or even some of his Zeppelin work. "The Rain Song" is essentially Robert singing over numerous guitar tracks.
I've watched people with acoustics and a looper sit down for 5 minutes and build sounds and song that sound like they were designed in heaven itself.
Pat Metheny is amazing at that, look at him play his Picasso guitar, its got and ungodly amount of strings he uses to harmonize with himself and create just awesome sounds.
An I've listened to Malmsteen play solo with delay and get some crazy sounds going due to the layering he can create.
Even if you look at guitar solos of some select artists they don't play so much against the band and will play di-note or tri-note phrases(inviting words here) to get sounds otherwise impossible.
I mean even in band tracks recordings of the guitars are often stacked in several harmonizing keys and then in octaves to give fuller sounds. Its one of the reason I shy away from modern music. Once artists start using "the wall of sound" it become near impossible to replicate them live without tons of equipment or multiple guitarists.
I actually have been given to using a harmonizer and octave pedal for that vary reason. I cut the volume of them both to half the main guitar sound and chose varying harmonization(3rds,5ths, ect.) It fills out my sound nicely, throw on a tiny reverb, cue my distortion and I'm good. Gets a little tricky when you want to use effect cause by that point your stacking a lot of shit but it can be done.
I actually have a few acoustic things I play with delay and reverb and chorus or rotary to create large swirling instrumentals.