Mm, hey! Thanks for the post, it has been quite interesting to read!
I would love to try out the Younger/Older story. Personally the first thing that came into my head was the movie Mercury Rising. In the movie, An aged, weathered cop protects a young, autistic boy who eventually helps with solving a key element of a crime case. Of course we can switch the autistic child for a young, teenage girl, but I rather like the contrast between veteran wisdom (from the police officer) and the prodigal intellect (from the younger character). Mm, Perhaps the young girl is highly intelligent, and is being sought after by the Crime family because she had unintentionally revealed some secret information that was dear to them.
I also liked the WW2 story, and as you have mentioned, it could be many ways! Or maybe a modern setting, a coalition of foreign nations fighting in Middle-East, and two soldiers meet >.>