Interesting thoughts I can't resist responding;
There are no "armour masteries" and no "superior" armour making it much more difficult to get armour class,
I'd argue the exact opposite. The only difference between Warriors and Combat Priests (half orcs usually) physical AC is typcailly the Armoured Dex and Armour Masteries and toughened skin. If Metal Armour is AC2 and anyone can strap on a set and there is only 1 rank of Dex (which will be more expensive for the priests I expect but presumably still obtainable) then I would broadly expect AC's to be the same for everyone. Until the priest gets hold of his +1 AC invocation of course at which point the warrior has to come begging
It's not only the spells and invocations that grant AC that would need looking at it would be the spells and invocations that do damage. How would bolts interact with skins/Dex/AC in general or cause mortals for that matter?
I am intrigued but wary, one of the things I like about HQ is complex system of which AC is a good example simplying it to this level would change the nature of the system to the point that it might be no longer recognisable.
I agree something has to be done to ensure that mixed rank dungeons can be run but homogonising the ranks so that they are all pretty similar seems to be counter-intuitive. "Come and play and develop a powerful character over the course of years" is a very different system model to "come and develop a character who will be slightly more durable over the course of the next few years"
I like the concept of some sort of "through" damage regardless of you AC (and this is coming from someone who has immensely enjoyed getting to the point that most of the party struggle to wake him up while asleep on certain missions!) but I feel it could be implemented by removing toughened skin mastery (replace it with another +1 physical perhaps) and changing the brusing rules to be 1 damage per rank if your AC would protect you to more than that point (this does mean that Quartz actually becomes scarier "base" damage as it will do a minimum of 6 not 4!)
My theory is this. The issue comes when you have high levels and low levels mixing it up together. In that case chances are fairly high that the low levels will have skins/protections running (whether guild assisted or party assisted, Gravey certainly wouldn't let trainees run around with a skin if he was around!) meaning that they are likely to be hitting AC10+ anyway meaning triple would be a reasonable amount of damage to throw in rather than single. High levels take 3 points of damage low levels take roughly the same, it's damn close to the 4 pts (from single) you want to see without having to over-haul the entire system.
Obviously the above needs work as I just came up with it while eating a sandwhich so I've not worked out all the comparative tables!
Shaun