I really hope Bergeron and Marchand don't get viewed as complementary players. I know pretty much everyone will probably have both on the wing, or separated as plug-and-play players somewhere in the line-up. At best they'll be put as Crosby's wingers to try to help insulate Crosby's play. It's still the Crosby show if you do that though. The play is always going to go through him. Why not make Bergeron/Marchand the centerpiece of their own line?
These two are arguably the best/most effective duo in the league since they were put together in 2010/11, and completely dominate the play when they're on the ice. They could be up against anyone as well, it really doesn't matter; they will outplay anyone. The play always revolves around those two as well, regardless of their winger. Whether it's an actual good player on their right wing (ie. Seguin) or a plug (ie. Connolly), their play isn't affected at all, and the player alongside them always ends up as the complementary piece rather than the other way around.
Obviously you'd use them in a defensive/shutdown role and would not be the line you put out in prime offensive situations, but they should still be a package deal IMO, and not as wingers to help insulate another center's game.
My ideal linemate for them would be Stamkos. He's someone that does not need the puck on his stick to do what he's great at (finding space in the offensive zone to get his shot off). Even if you send him out with the other two in defensive situations, they'll still carry the play to the other end of the ice and pin teams there for the majority of the time they're on. Let those two do the dirty work and just get open.