It's official - randomly generated shields on the loot tables are almost always terrible.
(It's official because I said it is , but I think pretty much everyone agrees with me).
Currently at high level, using a shield is actually detrimental to the AC of most AC-based characters (due to the Monk Wisdom bonus providing more AC than any shield can). As such virtually noone uses a shield beyond level 12, save for ornamental reasons or for occasional shield blocking.
Shields are meant to be about preventing getting hit and also damage mitigation, but they are outclassed in both tasks - better AC comes from being a Monk splash, and better damage mitigation comes from other items like the Docent of Defiance. Here's a couple of possible new shield modifiers that might make shields attractive again.
Lesser Elemental Resistance (+2 equivalent)
This shield provides the bearer resistance 3 to each of Fire, Acid, Cold, Sonic and Lightning (all at once).
Elemental Resistance (+3 equivalent)
This shield provides the bearer resistance 10 to each of Fire, Acid, Cold, Sonic and Lightning.
Improved Elemental Resistance (+4 equivalent)
This shield provides the bearer resistance 20 to each of Fire, Acid, Cold, Sonic and Lightning.
Greater Elemental Resistance (+5 equivalent)
This shield provides the bearer resistance 30 to each of Fire, Acid, Cold, Sonic and Lightning.
Superior Fire Resistance (+5 equivalent)
This shield provides the bearer resistance 50 to fire. (Similar shields should exist for all the other elements).
Improved Blocking (+3 equivalent)
This shield is designed as a bulwark behind which its wielder can hide in the face of great threats. When actively using this shield to block, the wielder gains an additional +3 blocking bonus to AC, and gains DR 8/-. (This provides no benefit except while actively blocking).
Superior Blocking (+5 equivalent)
This shield is designed as a bulwark behind which its wielder can hide in the face of great threats. When actively using this shield to block, the wielder gains an additional +4 blocking bonus to AC, and gains DR 12/-. (This provides no benefit except while actively blocking).
Giant Warding (+2 equivalent)
This shield was forged to provide additional defense against the gigantic weapons of giants. Against foes of the Giant type, this shield's effective enhancement bonus is +2 greater than usual, and the shield provides DR 3/- against attacks from giants. (Similar shields should exist for other monster types)
Greater Aberration Warding (+4 equivalent)
This shield was forged to provide additional defense against the otherworldly weapons of aberrations. Against foes of the Aberration type, this shield's effective enhancement bonus is +3 greater than usual, and the shield provides DR 6/- against attacks from Aberrations. (Similar shields should exist for other monster types)
Great Cover (+4 equivalent)
This shield was forged as a bulwark behind which its wielder can cower in the face of dragon fire. When actively using this shield to block, the wielder is treated as though they had the Evasion feat, even if wearing heavy armor. (This provides no benefit except while actively blocking).
There you have it - a few simple ideas that might well make a Sword&Board Fighter as effective at tanking as a Monk hybrid is, giving them both the opportunity to gain an AC high enough to be useful at endgame, or significant damage mitigation abilities. In particular, I'd expect +5 shields of Greater Evil Outsider Warding, +1 or better shields of Superior Fire Resistance, +1 or better shields of Greater Energy Resistance and +4-5 shields of Superior Blocking to be quite compelling reasons to consider using a shield again, for the many characters that have chosen not to use one.