I ran into this problem awhile back.... I had this cool song I wrote.... and then a solo just popped into my head. I had it all prepared.... then realized I had no rhythm background! Trying to figure out something for it was frustrating, I almost just decided to make it a pure guitar solo, no backing. A good trick is to play back the solo.... then fool around with chords and such. Power chords can be great too, as they are neither major or minor, it just depends the context you use them in.