Perhaps the simplest tweak is to allow ALL units to move 2 spaces, as long as they stop when entering an area containing enemy units, or which is enemy controlled.
Fighters excepted (maybe move 3?), and ships still subject to current rules regarding subs and transports.
This would at least produce some equality of mobility between the sides, though of course SZs are a lot larger and would still make control of the seas a big strategic advantage.
Maybe restrict movement in Africa to one space only due to lack of rails; also Arabia, Afghanistan.
Units at the front would still be restricted to a slogging match, but reinforcements could be brought up much more quickly. Might try this next time.
Might even think about tanks being able to breakthrough and attack a 2nd space with an infantry for each armour.
Perhaps, with the greater mobility, the suggestion that units can never move out of one contested area into another would make more sense.