I'd love to go on a U-571-esque mission in which an emeny vessel is captured and used to attack a major asset in its own fleet like a BB or a Carrier.
There's precedent for the capture of enemy vessels (Black Cat), and there's little chance of a sole sub successfully taking on such a target alone...unless it was one of their own. >:(
Additionally, in conjunction with my request in the land thread (ability to enter and sink vessels in enemy harbours at night), you'd have the best mission in E:RT possible: Get behind enemy lines, sink the target while docked, and try to get out alive.
I hope you can help East!