First of all, I don't usually like greatest hits albums. You usually add the so-called money-making and popular songs to an album and leave the prestegis ones off the list. Secondly, Greatest hits from Opeth would be a waste of time for Opeth. They don't have any hits. Every album is an epic with an album-orientated story. Most of the songs are too long for a greatest hits. For example, Blackrose Immortal is one of the best-respected and exhalted Opeth songs. I imagine any best hits from Opeth would have to contain this song, but it is fucking 20 minutes long. That leaves you with at most 60 minutes left. With those 60 minutes, you can prolly only fit, at most, seven more songs. That would give you a total of eight songs on a greatest hits from a band with seven albums; sounds like poor representation for a band. All in all, there isn't enough space to put a good amount of Opeth songs onto a greatest hits album. (They could do a double like the Doors greatest hits, but Opeth is not as popular as the Doors were).
I would rather have an album like Tool's Salival. It contians rerecordings of some songs, some new songs, and live recordings of some. Possibly Opeth could toss in a new cover or two *begins to think about a cover of Enter Sandman.*
Opeth just isn't popular enough for a greatest hits.