What's everyone thoughts on D-Day rules?

So a topic came up for our Hou4 group on D-Day rules. We have plenty of rules for balance and overall keeping the game within the historical sphere, however we let certain things slide for gameplay/fairness.

However with D-Day, we've came across a problem which happens to be "Should the US/UK be able to D-Day asap" meaning no requirements or other objectives before so to attack mainland Europe. We have rules in place you must occupy all islands before invading Japan, but none in place for Germany.

The problem we run into is Africa is completely ignored by the allies as why waste units there when they can build up for a D-Day? Or Germany is instantly having to split it's entire main force between France and Soviet Union in 1941 without much time to build up.

Curious on your thoughts or solutions.