Düsseldorf would be cool. Could be a good intro to the race with something like:
1) Düsseldorf prologue
2) Köln - Frankfurt (mostly flattish in the Rhine basin, maybe Mammolshainer Berg around 25km out)
3) Mainz - Trier (hilly stage in Rhine-Moselle area)
4) Trier, Saarbrücken or Luxembourg-Ville - somewhere in France (if Saarbrücken, we might be lucky and get something like Gérardmer)
OR
3) Frankfurt, Offenbach, Darmstadt - Heidelberg (stage including Königstuhl near the finish for an early intermediate stage)
4) Karlsruhe - somewhere in France (probably somewhere like Nancy, as if they do Königstuhl they probably won't want to do much in the Vosges)