The Americans did not make concrete plans for how to use the atomic bomb until it was clear that Germany would not be the target, because the war in Europe was coming to an end. I have never seen anything to suggest they made any kind of plans as to how it would be used in Europe had it been made earlier. As late as December 1944, it was clear to the Manhattan Project head that the bomb would not be ready for use in Germany, and the lack of American B-29 use in Europe would have added significant logistical (and national pride) issues that nobody came close to even thinking about seriously. Given that Kassel had been extensively bombed prior the summer of 1945, it seems unlikely it would have been considered a good target, at least under the target criteria used for Japan.