This one, I already discussed my feelings about in detail. I think it is too strong, and I think it is completely unnecessary to give more AC to shield users.
Necessary isn't the issue. It would be nice. Currently melee characters have two options: Train hard to learn how to fight effectively without a shield, or train nothing and use a shield. What is so wrong with a third option: training hard at being better at using a shield? That shield means a sacrifice of offense. It's not going to hurt anyone. What it will do is open up new build options and opportunities for people who actually LIKE the knightly aescthetic of plate mail and shield.
Since it seems I couldn't manage to get my point across, I will try to explain my argument in more detail. Because of the way AC functions, it has increased returns. The more you have of it, the better it gets, up to a degree after which it stays stagnant.
Take this example: If you are fighting against enemies that hits at a roll of 11 or above, that means you have a 50% chance of getting hit. If you then were to increase your AC by 5 points somehow, and only got hit at a roll of 16 or above, that means you have a 25% chance to get hit. That means that you are half as likely to get hit. Since you will, on average, only get hit half as much, you will also take also, on average, take half as much damage. In this case, 5 points of AC is roughly equivalent to 50% damage immunity.
And this is where the increased returns part comes in. If you are getting hit at a roll of 19 or above, then that means you are getting hit 10% of the time. If you somehow increase your AC by 1 point, then you would only get hit at a roll of 20, a 5% chance. This means you are, on average, half as likely to get hit and you will take, on average, half as much damage. In this instance, a single point of AC is roughly equal to 50% damage immunity.
In my personal experience, if you have more than 25% chance of getting hit, that is at a roll of 16 or above, you probably don't want to be in the melee in the first place, as you are likely to die before accomplishing much of anything. Taking that as an example, increasing your AC by 1 point in such a situation, your chances of getting hit would go from 25% to 20%, which means you are, on average, 20% less likely to get hit and hence you will, on average, take 20% less damage. In this case, a single point of AC is roughly equal to 20% damage immunity.
I believe that helps explain why even a single point of AC can make a huge difference, especially when you are facing late game content with very high AB numbers.
As for your other point, I don't see how this will open up new "build options," as you put it. Shield builds are completely viable, from the start of the game to end game content. Just today I fought in Sithicus with my shield using character, and I actually had a two handed weapon user and a dual wielder with me. I had +3 shield AC on my shield, which is readily available to everyone via varnishes and enchanting, and I was the only on who was capable of holding the enemies there in large numbers, due to my AC. Even though you seem to be convinced that shield builds are somehow unviable, I assure you from my experience with both playing a shield using character and witnessing other shield using characters in action, they are completely viable, even for late game content such as Sithicus, and they do perform their job of being defensive quite well.