The anthology is about the world ending and about what people are doing as that eventuality approaches. It is not about a Mad Max-style post-apocalyptic world. It is also not about the world not ending. There should be no Bruce Willis figures saving the planet. They can try, but certainly should not succeed!
The stories that have worked best so far are those (surprise, surprise!) that have most closely followed the guidelines laid out on this site. This sentence should be central in your thinking:
What we're really interested in is what ordinary and extraordinary people will be doing as the last grains of sand run out.
And don't hesitate to contact us should you require clarification on something or to make a query.
Once again, have a wonderful Christmas and, without depressing your friends and loved ones, do try and think about the end of the world! :-)