Back in the day when I used to play really heavy stuff, I wanted to get away for the power chord song writing, I listened to Death, Slayer, Yngwie, stuff like that and the thing I noticed was alot of their melodies were just harmonies. Usually thirds, and in a minor key. Once I figured this stuff out in the beginning, the songs and melodies and riffs, just started flowing. Not sure where you are with skill level but try recording your self playing a C maj scale, then move up a third to E and play all natural notes along with the C maj scale and you will hear the harmonies, then you can try variations of that for dissonance. Hope this helps