I brought this up a while back, but here I'll be taking a new spin on it.
A succession of treaties in the nineteenth century established Luxembourg as a neutral state between two great powers, dismantling a vaunted system of fortifications.
WI after WWI, France, Belgium, and Luxembourg met and decided upon an extended, fortified line (a larger Maginot Line from OTL) with Luxembourg as the line's lynchpin, reviving the country's history as a fortification center. In theory, the line would serve as a tripwire and chokepoint for a future invasion, allowing time for mobilization. In this TL, Belgium does not pursue neutrality and French, Belgian, and Luxembourger armies maintain a decent readiness throughout the Line.
Does this change anything about WWII? Do the Germans break through with the ease they did in OTL, or does a larger, better-prepared defensive line hold them up or even lead to their defeat? As France would most likely be the largest contributor of soldiers, would the existence of this line have a positive or negative effect on the mindset and morale of the French military compared to OTL?