Pitchshifting Dual Delay
Set different harmonies for the left and right channels. For example, -12 will be an octave below, and 7 will be a fifth (7 semitones) above. The Balance control can be used to emphasise one side over the other.
Use Delay and Feedback, along with LFO and Frequency, to set the length and character of the delays.
For more complex arpeggiations, try using two instances of Angel Dust at the same time.
http://witchpig.co.uk/plugins.html#angel
