How about instead of making the trench a garrison, turning it into the same type of trenches that are found in some maps, (like the ones on rails and metal, that are on the north cp and south fuel point) these however would allow vehicles to move over them to prevent players from blocking sides of map with them
Right now they are pretty useless due to all the counters, flamers, nades or mortars both ally factions can get any of these, some trough doctrines (like US mortar halftrack with white phosphorus)
Would also like it if this type of trench building came with ambush camouflage on the same doctrine
I like this idea the best so far. At least your units wouldn't be force to be static inside like they are in garrisons.