With no third, power chords are great for modal music. In traditional Irish music acoustic guitarists use them a lot. A real popular shape is 079900. It is used with tunes in Em. Start on 079900 then slide the shape to 057700, to 035500, to 024400 and end on Em 022000. You can use these in many combinations the low E gives the drone modal sound. They sound real good picked as well as strummed. There are many acoustic guitarists here who base there style on a complex system of power chords.
Derm