The major impediment to using artillery and mortars (and other heavy munitions like bombs and rockets) in Arma 3 is how overpowered these things are vs infantry, even if buildings are present. This is especially a problem when players were on the receiving end of it, but even when used against AI it led to situations where it was too easy for players.

Fundamentally, the problem is that there is no good ways for infantry to shelter from artillery in Arma.

Having "proper" trenches, earthworks and fortifications would enable scenario makers and game masters to work artillery more effectively into missions, and make it immersive yet survivable when used against players. For coop scenarios, it would enable players to use artillery for thinning out a position but will still require them to clear occupied fortifications.

These do not need to be dynamic, but just having editor / Scenario Framework / World Builder-placed trenches and foxholes with baked in terrain deformation, or a way to deform terrain via GUI and put in trench objects, would be really good to have. AI should be able to navigate these structures and occupy positions in them (like with bunkers in Arma 3).

This would of course require that artillery (and in general HE rounds) are simulated in a way that trenches and foxholes actually work, in terms of shockwaves and/or fragmentation and shrapnel.