Hamburg
I am aware of Dusseldorf being a hub in it's own right but as Hamburg is not currently served from Southampton and the fact that many passengers fly from Hamburg to Heathrow to connect with the cruise liners from Southampton in particular Cunard Lines Queen Mary 2 I added it to my wish list of destinations.
On August 04th 2010 the Queen Mary 2 had just under 350 German passengers onboard with just under 250 of those arriving from Hamburg to Heathrow!