That one's difficult because I've already read a few fics like this and I'd want to do something different. So to start with I would probably kill Nate and Eliot. Nothing personal, but they'd be the best at dealing with the situation and the most likely to die in most apocalyptic scenarios. How about the Wraith? Or similar aliens, or whatever. Hardison, Sophie, and Parker are somewhere where going out is very dangerous. Hardison tries to repair his computers to make contact with anyone, Parker goes out to get them food way more often than she should because it's dangerous and she's wounded, but she'd go crazy otherwise. Sophie doesn't know what to do, so she tries to look after Hardison and Parker even though both of them don't need and don't particularly want it. All three of them are terribly at grieving. It would be a small fic, only background plot, that's mainly about dealing with their grief alone and together while facing the uncertain future.