Okay, here are my ideas so far:
- U boat refitting at sea from "trawler" type supply ship (requring some open hatch conversions)
- U boat filming survivors in life boats (easiest and one most likely I'll actually get to)
- U boat in dry dock in France (a la large Andrea diorama)
- U boat being taken over by boarding crew (requiring lots of figure conversions)
And of course, the one which someone with more space and patience than me will build...
- Flower Class Corvette versus U Boat