Rhine-Ruhr is Germany's megacity. Perhaps Essen would be the best place for a baseball stadium? Being regional, perhaps the best location around the Ruhr would be a balance of entertainment, restaurant and lodging options very near a highway. Or a vacant spot could be spurred on to propel growth in an area relatively central to everything.
If you had 2 spots to play home games for the representative squad for Germany, Berlin would be a great place also. 3.4 million people are there. 1.3 million are in Munich to the south, if one were to consider 3 camps.
I believe there is plenty of love for baseball that is ripe for the Rhine in the coming decades.