There is a very big discussion over the years abut airborne(able) new units (paratroopers, mostly).
But, instead of that, and have to add a new unit, how about to add a new feature? A new feature to air transfer the units? maybe at a level 2 airport. you can (air)transfer a (stuck?) unit from the one airport to an other airport, say at a 40% of the total(=the actual road trip) time?
I think that this is the right place to post this.