Maybe limit the number of combat rounds in any battle.
Anyone tried using the 1914 rules in WWII games? It would make sense to have more than one round as per 1914, since warfare in 39-45 was much more mobile.
Also/or allow defender retreats after a set number of rounds; armies rarely stood where they were until annihilated unless surrounded as at Stalingrad.
1914 contested area rule might break up superstacks, since it takes relatively few units to make an area contested (and deny the defender income).
So how about:
Use contested area rules, but combat can go on for 4 rounds. After 2nd & 3rd round defender (then attacker) can choose to retreat. If area ends up contested the defender can still build units there (if eligible) but collects no money.
Having a limited number of combat rounds might also encourage the building of mixed forces, since large infantry heavy stacks would have more limited attacking power.
Incidentally, I don’t think aircraft should be allowed to land in an area you just attacked and made contested.
Amphibious assault rules would have to be modified also.