Ive done a few Diadem HMs & in about half my party has been the only one in the zone, which makes spawning & killing the star rank mobs rather difficult.
My idea for HM is when a party would get put into an empty instance instead put them in a queue & then drop them in once maybe 3 partys are ready, that way no one ends up in a dead instance wasting 2 hours
I also think that the way partys are distributed among the instances may need to be tweaked a little.