Grep, I'm not sure if I agree with that, yet. I consider the amount of active players at any given point in time the fundamental source of pleasure and entertainment (retention factors). When there are enough players they can do whatever they want with the challenges that already exist in Pyrathia, or make up their own challenges sociologically.
Your suggestion speaks to new challenges that would take advantage of large active player counts. I think there could be new areas or challenges built to accommodate this, but I also think that it is more important the Player-base have enough different elements (roles) to enjoy in the first place.
So what replaces the high-count bonus is the regular group-bonus to melee experience, and the difficult-to-measure increased player interactions in general.
Good picture to describe this process
here.