I don't see many threads on this topic. Basically, I am in a band that more or less could fall into the metalcore genre. The other guitar player in the band told me that basically I should focus on clean and melodic parts and he is coming up with the rhythm. I realize that trying to write songs like this may cause the songs to not fit together all that well. Often when I am trying to come up with a chord progression it either 1) doesn't sound very metal or 2) doesn't sound interesting or captivating. So, I need some help with chord progressions for the clean parts of a metal song. I know quite a few voicings and variations of chords.

Raconteurs Fan Guy
The biggest thing is play the notes individually instead of strumming chords. Metallica does this a lot. Obviously they're not metalcore, but that's the first example I could think of.
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Check out Def Leppard. They were past masters at this. Not metal, but nonetheless.

cheers, Jerry
Arpeggios are your friend! Also, pick more interesting chord than just your standard major, minor, and diminished triads. Add notes for coloring.