Would you include german coastal defence ships?
http://en.wikipedia.org/wiki/Siegfri...l_defense_ship
http://en.wikipedia.org/wiki/Odin-cl...l_defense_ship
I know that they were considered useless even before the war and were withdrawn from active service as soon as 1915 but since then they were used in ports - mainly as barracks ships. They are pretty unique among the german ships and could add some ambience to the ports.
EDIT:
Also, how about creating a special program like SH3 Commander which could maybe allow player to get many awards from many countries and other things (like banner from a sunken ship maybe) based on your savegame, manage your crew more easily or even interact with them? This would help overcome some hardcoded limitations perhaps.