I didn't mean to imply you shouldn't have some structure.I guess every guitar players dream is to have songs with melodic lead parts that are integral to the music(to the point that people know them note-for-note).You don't want to depart from the melody,especially in an instrumental setting.Having said that,I can tell you I've read plenty of interviews with popular guitarists who never play the same lead twice and,in the studio,don't plan for the lead and just "go for it."In blues and jazz,you see a lot of improv,but that's not mainstream music.Pop on the other hand is formulaic and repetitive and people always expect it to be exactly the same.I'm afraid I am not a jukebox.