Altough that sounds like a good idea, but as mentioned before, Kalmah shouldnt cover a song with the same genre (black/death metal). If Kalmah covered a Gorgoroth song, it wouldnt be any much different from the original, (besides the increased ass-kickingness.)
I think any metal song from the 80's should deserve a cover more than any other song. Take Dimmu's cover of "Burn in Hell" as an example. And because Kalmah's melodic sound has so many influences from such classic gods as Iron Maiden and Judas Preist, a cover by one of those bands would probably be best in my books.