I wanted to offer a suggested game mechanic my company and I were discussing the other night.
Currently players who carry a shield on their back but do not wield a sword/shield receive weight penalties while not receiving any benefits of wearing a shield. Additionally, Sword/Shield users have their load capacity penalized over that of other weapons solely for the fact they carry a shield.
This suggestion is to add purpose to the shield even when not actively used for combat. While a shield is equipped on a players back, that shield should provide some degree of damage mitigation/protection from backstabs. To take this a step further, the larger the shield, the more protection that shield should provide.
This concept gives players more options for their builds. For those not wielding a sword/shield as one of their weapons, it offers the option of carrying a shield anyway for added backstab protection at the expense of upping their total carrying load (and thus weighing them down more), vs. not carrying a shield (and thus a lighter carrying load) but being more susceptible to backstab damage. For those that do wield a sword/shield, that backstab reduction should only apply when the shield is on the players back, not when the shield is actively being utilized in combat.
As an added concept, Tower shields should provide some form of head shot damage reduction due to their size and the concept that there would be a chance that shots fired at a persons head while they are holding or wearing a tower shield could be ‘deflected’ by the tower shield itself. Since I think full ‘deflection’ of headshots simply for wearing the shield is extreme, I believe offering a headshot damage mitigation or something to that effect for headshots would provide more motivation for players who both wield a sword/shield, and who do not, to make the decision as to whether or not they should carry one of these items on them.