If you've ever watched Kelly's Heroes, you know it would be a good premise for an MOHAA add-on. There's gold stored in a bank behind German lines - Clint Eastwood and a few other greedy GI's make a run for it. On the way, they tangle with Tiger Tanks, friendly fire from Allied planes, a bridgebuilding exercise and more.

One other thing I'd like to see is a scenario with a massive German counterattack, something like the Battle of the Bulge - maybe on a smaller scale though.

As others have noted, the South Pacific island battles are good too, especially Guadalcanal and Iwo Jima.