During WWII, Sherlock and John were captured by Nazi soldiers because of their relationship. Either they grew up not English or Sherlock was a translator and John a soldier. John a sniper and Sherlock a spotter.
-A story where they escape from the camp, are on the run, and hopefully find freedom, by finding Allied troops or they make it to England.

I also like wump John, especially if he's trying to save Sherlock from some of the horrors while in the camps or during escape