Implement weapon collusion with walls and other obstacles. Apparently the weapon resting must already "be aware" of the area immediately in front of the player's weapon, and the FFV has an animation where the weapon is raised when it hits the limits or an obstacle.
This addition would have a tremendous effect on close quarter battles. It would make SMG's and short carbines viable. It would make a bullpup weapon trump a traditional weapon because if the size. It would mean that team composition would wisely include at least one member with a shorter weapon for room/house clearing.
People like to argue that Arma 3 is not a CQB game, but this is one of the reasons why it isn't, and one of the possibilities how it could become one.