I think the better solution to having both HUGE maps and positional play is to have transports be a staple of gameplay.
Transports such as flying transports can potentially be much, much faster than land units, and also gives players a very compelling reason to control the skies. Air superiority actually makes your units more mobile by allowing friendly transports to operate safely, and reduces enemy mobility by destroying enemy transports or preventing them from entering the area.
Transports are also a very valuable asset that is not, by itself, a direct combat asset. They don't directly attack the enemy, although they do convey a tremendous advantage to your other forces.
Building a large number of transports consumes resources which might have been spent on more combat forces. However, having many transports makes your forces more mobile on a strategic level. A large army with relatively few transports is undoubtedly very powerful, but has limited ability to project that power over significant distance, and must move very slowly.
This also creates an interesting decision regarding the strategic movement of units which is different from their tactical movement when they are driving around on the ground. Deploying units using transports can potentially move units great distances quickly. However it occupies a limited resource (transport time) which could alternatively be used somewhere else. Perhaps to transport other units to somewhere else, for example.
Fast transports also allows for practical movement over enormous maps. Those very slow land units will just depend on hitching a ride on a transport in order to cross the entire map. It's fine if the unit would take an impractical amount of time to drive the entire distance by itself.