The promos look like the humans need to be protected from the wesen. I am thinking the mirror world is a reverse of this one. over there wesen are in charge and humans are the outcast. Because the death grip was of a wesen that died here but was passing to the other side. I would think the blutbad Nick killed in episode one will also be inside the mirror world. I think meeting him there will be why they have to go back to the blutbad cabin from episode one. I also think that saving the little girl in that episode is what puts Diana at risk.
Embrace your inner Biest..... We all have one