I'm still a newb at this game but alot of posts I've read say that defences are underpowered, and that is certainly my experience in game.
A possible solution to this would be a mine layer.
If a specific vehicle can be made for this role, it could be tasked to plant mines in an area similar to the way a con is tasked to recycle an area, the vehicle will then work away automatically, however once a vehicle has deposited a specific number of mines it would have to return to base to restock them before carrying on.
A specific building could be made necessary for restocking mine-layers, this building could be required to use the energy grid so that "free" extra mines are mitigated by the cost of powering the re-stocking building, and the fact that its connected to the grid means that it is much quicker to lay a minefield by your base than by your opponents.
Obviously alot of balance testing would be needed for this new type of unit, and I believe that the initial unit cost should not be to cheap (to avoid spamming the whole battlefield with mines), also the damage / number of mines held would need to be carefully worked out i.e. how many reloads to deliver enough mines, to cause enough damage to recover initial cost of vehicle.
Also I believe the vehicle should not be to fast, also making it easier to mine near your base than farther away.
some other variations on this theme could include :
a) A fast kamikaze mine layer that drops its mines when destroyed. ( not too many mines and not to cheap ) an alternative to the walking bombs.
b) A mine detector, every unit can destroy mines by shooting, but only the detector can see the mines, the detector should be given only a short detection radius, and a slow speed, because linked with another unit (guarding it) the detector could be set on a path and the guard would take - out all the mines on the path.
c) The mine layer could double as a mine detector.
d) An Expensive bomber that could drop mines instead.
There are many interesting variations that can be made with this idea, in my mind hopefully it will make it harder to flank defences, and will just add many more interesting tactics to a good game.