I can't comment on how hard or easy things would be to code.
The idea to change from a free roaming army to one that uses roads is simply to avoid having to program pathfinding. I wouldn't want to see a convoy of tanks and trucks driving overland just go up a sheer cliff, or bother trying to program how they veer around trees and such, or cheating and just let the AI units ghost straight through obstructions as if they didn't exist. If you can only follow roads, you have a lot more control, and then it just becomes the problem of the guy who has to make the map to not make a silly map.
I would assume that mobile army groups would absolutely have a place they start from, which should probably be something you can capture. After that, the army marches out to the front, and you can route it down the fixed pathways of the roads grid to where you want it to go. The major crossroads locations would be the capture points of the game. Towns with an airport. Big towns with big airports, ect. When it runs into the problem that the next location on it's route is enemy held, then it stops just before entering the crossroad location, converts into an immobile army camp of tents, parked trucks, foxholes with AA guns and AT guns, artillery guns, and player vehicle spawning is enabled. Then you jump there, pick your favorite tank, and spawn on site and drive over the hill to take down the enemy AI guns and bring in the 10 troops.
Surprisingly I think this might actually both reduce the amount of tanks in use overall, but paradoxically ought to increase the numbers of them seen in the immediate vicinity. You would only see players in tanks when there is an active siege to take a crossroads town with it's obligatory airport. It's reducing the
If you want trade convoys to attack, well just have pathways that cross water. An army could cross an ocean by going to a port, then be sent across the ocean, but now they are in the form of a convoy of cargo ships going along a convoy route if you will. That might make U-boats worthwhile in the game, as you could spawn one and head to the likely routes an army will be crossing the ocean, rather than searching the whole ocean in vain.