I'm unsure how possible/feasible this idea is with the current tools that we have but I would love to see a mod that alters the position of holstered weapons; with two options which would make the holstered weapons on the back more symmetrical.
One option being that they are both held under the shoulder like the left shoulder weapon, so they would look like |o|, where the circle represents Arthur; and another option which has both weapons over the back like the vanilla right shoulder weapon, so the weapons would form an X like shape on the back.