I think simple addition of prefixes and suffixes thematically oriented towards shields would be nice.
Currently, shields offer somewhat varying bonuses depending on tactic settings. For example you don't train shield marks while on berserk. I would like to see stronger connection between tactical mode and shield effects.
Some features could be shared with armors - extra dv/pv/resistances etc., but also a few shield-specific things, such as:
*foo* of mirroring - reflects a percentage of physical, melee damage at the attacker.
*foo* of bashing - allows to add the shield's weight (after recalculations of course) to damage output in berserk tactical setting. Would be the equivalent of hitting things with your shield, greatly increasing the chance to break but offering some last ditch chance in situations like being disarmed or wielding a cursed, broken weapon.
*foo* of containment - every time the PC is hit by a melee attack, a small amount of HP is regenerated. Could be extended to cover magic bolts which would replenish PP instead.
*foo* of wind - gives the PC a speed boost at defensive tactical settings - the closer you get to coward, the more speed you gain. Could be a nice variation on existing concepts of battle strategy.
*foo* of attraction - greatly influences which item gets destroyed first in case item destruction occurs - traps, eyes of destruction, annihilators, environmental hazards - all this by focusing damage on itself. Depending on shield material, some could withstand more punishment than others.
Jagged *foo* - causes monsters that wield weapons against the PC to occasionally get disarmed (generating a random, thematically proper weapon in the adjacent tile, lowering monster's damage output...).
Glittering *foo* - essentially the equivalent of an amulet of light but in shield slot, with otherwise regular shield bonuses.
Distorted *foo* - causes hostile monsters in melee range to have small chance to get confused for 1d3 turns, looking at the misshapen shield, giving that precious moment of respite when you're in need.
Vibrating *foo* - in defensive tactical settings, works much the same way as "of searching" feature - the justification being that careful PCs will explore dungeons with their shield held in front of them.
Just a few things off the top of my head, more to illustrate the concept than give actual ideas.
I realize most of the stuff I wrote above would require some serious coding, testing and debugging, with small overall game improvement, so probably not worth it, but a man can wish.
Grond would be happy I'm sure, finding all the ways these modifiers aren't supposed to work
"Hell is empty and all the devils are here."