I remember I made my own Bunker Modules years ago. Outposts, Pillboxes, Towers, Sandbags, Dirt Tunnels and dirt patches to "rebuild" damaged terrain.

I lost them a long time ago when I switched laptops but maybe I could bring some of them back, specially the pillboxes. I could even make modules that can fit tanks inside. The AI won't use them but this is more for players. Lets make a list of possible modules:

Pillbox: Small concrete/metal guard posts with a small exit/entrance at the back and bottom so players can connect them with the main base or just close the entrance with a piece of concrete.

Sandbags: Cheap cover usually set up outside bases. Will range from the usual front cover to small tunnels and sandbag modules.

Tank Bunkers: Modules big enough to fit Tanks. Will range from Tank Guard Posts to Hallways and Housing Modules.

Faction Specific: Stuff like Banners and Propaganda Posters. Maybe random stuff like piles of infantry equipment or furniture.