Have them detect a human lifesign and have to go rescue her because the closet of the mostly dead in the ship of the dead is lined with a substance that reduces smell but also makes it impervious to transport, or something.
Speculation on next season, kinda spoilerish since it involves some information from magazine articles and not the show itself:
Spoiler!
I guess Frakes and others let it slip that the Mirror Universe would be involved in some episodes, and there's a photoshoot in Variety from the bridge:
Which if you zoom in on the plaque shows a definite difference:
So they'll meet up with the alternate Enterprise it would seem!