Hamburg possibly. Their GFL team (German Football League) is the best supported among the cities they might consider. Of course judging from GFL support Braunschweig should get a team (not that this will ever happen, thankfuly).
It's a shame, but I think the dwindling attendance shows why it happened. It might also make it easier as far as travel expenses and to try to keep a league in a more concentrated area. It also just seems to me that Germany seems to be more receptive to American culture than other European countries. Whether it's because of post WWII conditions I'm not sure. Just as soccer seems to have a lack of interest in America as a whole, I think there is a lack of interest in Europe in American sports such as American football and baseball.