One thing this thread has made me think about, that is somewhat related, is a comparison between how the Germans went about defending Western Europe against invasion and the US doing it.
Now, the US spent years thinking about how to defend a long coastline. Their conclusion was to make the major ports defensible against direct invasion while producing mobile systems to defend the remaining coast putting nothing or next to nothing into fixed defenses. Also, the US figured out what seems reasonable methods to minimize the vulnerability of their weapons and, which ones might be most effective against naval vessels.
What if the Germans had come to similar conclusions? That is, they built more hard to destroy open mount heavy guns with good protection for the crew when under fire and mortar batteries (after all, they had a good number of very large mortars in the 11" + category available) that were invulnerable to naval gunfire (even if somewhat vulnerable to air attack) while concentrating on improving road systems laterally behind the front.
As it was they were using as many as 250 rail cars of cement a week building the Atlantic Wall not to mention thousands of tons of steel. Putting a fraction of this into railways and roads while continuing to build cheaper coast defense gun positions might have gotten them a better defense. How would things have changed in Normandy if the Panzer divisions could have moved there in, say, half the time it originally took them and, they arrived complete not piecemeal?
A mobile defense in depth backing a system of very heavy coast defense guns and strong points laid out to prevent taking critical points like ports (note: the coast defense guns would have a 360 degree field of fire like many US guns did so they could support a land defense not just fire on a seaborne assault) might have made more sense. While some of this is the von Ruendstadt (sp) concept, adding a deliberate increase in roads and rails was not part of the original idea.