Well, seeing as Per is permanent (not just for touring purposes) and Mikael says on the dvd that he has some ideas that border on black metal moreso than death, I actually am curious to see what it would be like if the keys had a little Dark Tranquillity style (though not as pronounced and synthesized) for small sections, and perhaps maybe a slightly Old Man's Child sound (used exceedingly sparingly, and of course with the trademark Opeth chordal shifts and intervals). Really, though, what I like about Opeth is that it doesn't matter what I think. They'll do their thing and it will blow me away. Everything they do just has that feel and sound.
Also, as a random aside I would love to see a version of Closure where it was all heavy (with their usual distortion) except for the 2:45 - 3:30 section. I've always felt that of all the Damnation songs Closure's first two riffs and the last section are the most reminiscent of heavy Opeth, like they could have done it heavy if they wanted but just chose to do it clean. Since they write everything on acoustic guitar first, that might not be so far off.