Strictly speaking of rhythm guitar (although lead techniques can help your rhythm tech. as well), I would focus on alternate picking quickly--try patterns with rests in strange places and such, so that you develop accurate and focused picking skills rather than just the ability to play sixteenths for 4 bars--, artificial harmonics (as well as natural harmonics), economy picking for the odd run, maybe small sweeps (3 strings, maybe 4), a little mild tapping, and of course, string skipping. Then combine all the techniques.
I make up my own exercises, so I would just focus on learning these techniques with scales (for instance, to work on artificial harmonics, try making the harmonics play a specific scale, etc.) and chords for the sweeps. But again, sweeps are probably less important than some of the other stuff. String skipping is probably one of the most crucial, along with alt. picking, just because it frees you up on the fretboard--oh, and don't cheat (no chicken picking).
I was under the impression that you already played (?). In any event, best wishes in your metal studies.