well honestly, i thought the "Breaking The Silence" duet was good, but its not one of my favorite songs. like i'd only put it on Firewind's Greatest Hits because it was one of their singles.
well my favorite metal vocalist is Damnagoras from my favorite band Elvenking. not sure how a duet with that band would work, but i guess it would be interesting to see what happened.
in rap they always do duets and feature other rappers, in at least half or more from each album. Metal doesn't really need duets. its interesting a couple times per career, but i don't think Firewind needs to do a duet for a little while.