Hey guys. Kinda frustrated at the moment, and its at something so ridiculously simple. Im writing a sorta techy rock/metal song, in which the lead synth is playing this riff which uses only the notes in D minor (D,F and A) The majority is just a D note played in rhythm with the A note played at the end, and the bar after that ends with A then F. This phrase is played 4 times (thats 8 bars worth of D minor-based riffage). The problem is, i want some awesome metal power-chord riff under this, but i just cant figure anything out which doesnt sound like trash.
How would i go about finding which notes to play on guitar over this? Its going to be rhythm gitar, becasue the piece really needs a good metal guitar rhythm, but how would i go round this? please be easy-to-understand with any theory too please. I know a fair bit of theory, but some terminology still deludes me.

Anything using the D minor scale... Try using Drop D for some Brootal riffage?
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As my old guitar teacher once said: Metal really comes from classical music. The only difference is pinch harmonics, double bass, and lyrics about killing goats.
Drop-D, open, hit chords that are in the scale you're using until it compliments the synth.