Wearing the Mitre of Holiness removes the spellcasting penalty from wearing a shield.

Stone shield, a magical shield made of stone that provides stoning resistance, but is quite heavy.

Conceptually separate breath weapons from rays, making them not trivially reflectable. Make it possible to suffer reduced effects from a breath weapon, with the size of a worn shield a factor in that reduction.

Possibly make dragon armor of the same type as a given breath weapon (e.g. red dragon armor versus fire breath) reflect that breath weapon, and only that one.

You can apply an equipped small shield (or possibly other types of shields) to attack a monster with a shield bash. This could be treated either like a blunt weapon attack, or more likely as a special effect which has a chance of stunning the target. Stunning odds could be higher for either more martial-based roles, or could use a shield skill and scale with that.

When you switch from a one-handed to a two-handed weapon and you are wearing a shield, you are asked if you want to remove the shield, and if you say yes it is automatically unequipped before wielding the new weapon.

If you aren’t wearing armor or a shield and have high martial arts/bare hands skill, you get an AC bonus (probably a higher one for martial arts). The bonus should probably scale up with higher skill, and should be generally higher for martial arts users than bare hands users.

You can polish metallic shields by rubbing a towel on them. While it is polished and erosion-free, it provides reflection; but once it gets hit with some form of erosion it loses its polish and can’t be polished unless the erosion is removed.