Sorry if this has been suggested before!
I think that the game could benefit from a land unit scramble option into adjacent land zones from cities or a FOB.
I noticed a potential need for this when I was playing Germany and I had fighters in Paris so that I could better defend the French region. I was annoyed that I couldn’t have my armoured units (mech, med. tanks, etc) stationed there like IRL to respond to a seaborne invasion.
I think the rule should be either:
Up to three armoured units (or units with a movement of 2) can scramble out of a city to defend an adjacent territory. or;
Instead of a city perhaps a FOB can be added into the game to act as a land unit airbase for scrambling purposes, or even movement purposes as well (I think that the game is too short and land units movement needs a buff, perhaps +1 movement from an HQ could help).
I think this would add quite a nice dimension to the game. Making cities more important and making it easier for nations to go on the offensive by not needing to stack as many units on every territory (think Japan on the Chinese coast).
Maybe the incoming scrambling land units can defend at a -2 on the first round of combat (minimum 1 defence) to simulate them coming late to the battle (not as fast as fighters).
Please feel free to critique this or add to it!