It could work as kind of gamble. You can choose to block with a shield with the mechanics we have now, or another button for "manually" blocking with a shield. If you block it succesfully, your shield suffers reduced damage from melee attacks, and if you fail, your shield will take inreased damage.
Add some time window for this to make it more difficult (cause failing at normal manual blocking costs you hitpoints, but this only damages your shield). Maybe so that when you click the "manual" shield block, you have like 100ms of reduced damage, 100 ms for normal damage, and if the timing is off by more than that you'll suffer increased damage to the shield. The whole duration of this "manual shield block" could be something like 400ms, so once you choose to go for it, you'll be stuck with it for a moment. And this way you can't bind a macro that constantly refreshes your block, thus giving you a "manual" block and a normal shield block at the same time.
It could actually be very similar to chambering, the risk as well as reward is just waaaay lower.
Just and an idea I got from this
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And about Harpag's suggestion:
Not sure how this blocking with a weapon and with a shield would work. Basicly you could just turn your shield into a passive arrow catcher, while you're fighting otherwise identically as a shieldless 1hander? With the exeption that you can start using your shield as you please (when getting ganged), without a normal penalty of taking it from you back? Sounds kinda Op
And I don't think it could be possible with current mouse&keyboard machanics / brain I have right now to control each hand separetely. I mean like blocking a sideswing with your shield at the same time you're blocking an overhead with your weapon. If this is what you even meant in the first place.