Since we are asking for expansion to a sub game to be a whole new game and not just an expansion at all (heck, we don't even need subs in our subsims!), I would like an expansion that allows me to drive an Auto Union at Nurburgering in 1937.
Controllable destroyers would not be in an expansion.. that would be a whole new game... why don't you keep expectations a bit more realistic, say a Fleet Boat - Atlantic expansion... ya know, maybe submarines in a subsim? DC was a stand-alone game, not an expansion.