A useful structure for engineers to be able to build would be boards. As with the board destructible in the game, a few shots or a single axe hit would break it. It’s unique usage would be that it can collide with cover buildables and walls, as well as being buildable as long as it is colliding with any buildable/terrain/structure that is not a strategic object or weapon.
This could be used to provide additional cover for certain engi structures at the cost of visibility, blocking up windows in a less obvious (and maybe faster?) way than sandbags, or even quickly blocking off doorways.
Just use sandbags if you want to seal a window from small arms.
It seems unnecessary and even counterintuitive given that oh-so-ubiquitous rifle rounds can easily go through 1-2 boards of wood whereas a stack of sandbags is a good visual indicator that you need something bigger than a rifle to go through it.
Not to mention its pointlessness when sandbags can already protect you for 1 unit of engineer resources and can be built in 3 seconds, whereas what you’re suggesting might as well just be a regular map prop.
This isn’t CoD Zombies. Boarding up windows is pointless when you’re fighting soldiers armed with guns.
The big issue with using sandbags is that it is very obvious, and unless it’s at a control point they will assume a rally point is in there and want to flank it. Also, blocking a window with sandbags doesn’t allow you to shoot out of it (except a few windows, but then you aren’t getting the cover of the window).
For example with a machine gun nest you could board up the top of the window above it, which sacrifices a lot of your view, making you more vulnerable to enemy MG fire, but makes you much harder to hit by snipers or flinch shots.