Ole von Beust apparently wants to create a Northern state from S.-H., HH and McPomm - I doubt that's a good idea.
However, the three federal states together would have an even greater overrepresentation of rural regions compared to urban regions in terms of population numbers. And thus, the prospect that this Northern state would firmly be in the hands of the Union - despite a few larger red cities and despite Hamburg. Presumably, that's the simple main motivation for them to calculate something.
However, whether the people of Hamburg would give up their independence and not simply vote out this eccentric at the next election is another matter.