I got an idea regarding shield mastery. It's only related to shield bash in the way that shield mastery grant shield bash.
What if shield mastery granted the ability of blocking arrows with your shield, WHEN not wearing a shield OR a bow AND having a shield in inventory.
Basically it would auto-missile shield when not using a fired weapon. This because in my opinion wearing a shield never pays up.
It would be "Jonny the warrior quickly seeks cover behind his shield."
So, you suggest allowing a warrior to essentially put a shield in the fired weapon slot and have it block other fired weapons, but no other shield bonus? That means you could two-hand/dual wield and get arrow protection at the cost of your own arrow output.
It is a straight wash (your arrows for my arrows), but it could be a tactical decision. Tactical options are fun and we all know warriors could use some variety.
A shield master warrior means they have no weapon mastery. Therefore, allowing the shield in the fired weapons slot means they will still be below par of every other warrior (since every other warrior would be allowed to use a weapon lore and have a weapon prof advantage -- the mutual arrow loss part is a wash). So, I don't see any major balance reason against it in basic theory, but I will expand more on this later.
In practice, this will have the most effect on a shield warrior who wants to use a non-shield combination against a fired weapons user. The main scenario that jumps to mind is a warrior vs ranger/ninja. For rangers, the warrior wants to use a polearm and nullify fired weapons (if they were vuln for instance). Otherwise, the warrior would be reduced to mace + shield to get the same effect. The difference between polearm output vs shield master + mace output is probably in favor of the mace/shield, but could be a tactical consideration to use a polearm for some reason. I see no big balance concern here. Ninja I will talk about later.
The other scenario would be when shield warrior is clearly outmatched in arrow dps (such as vs watcher or a non-returnable vuln). This will strengthen warriors vs watcher ninja for example. I do not think that warriors need a buff against rogues in any way. It will also help dwarves/duergar, feral, and fire giants cover their arrow vuln.
Shield Warrior Advantage (with this change):
Allows polearm against ranger
Allows dual wield/two hand vs ninja
Vuln arrow nullified
Shield Warrior Disadvantage:
Cannot use shield lore or shield prof advantage (since no shield)
Must sacrifice another warrior lore + weapon prof
Lose fired weapons DPS
So as a net result, the shield warrior will accept lower output (no warrior bonus prof/lore and no fired weapons) for the advantage of weapon type flexibility (already enjoyed by all warriors) and the ability to stop opponent fired weapons. Will this make shield mastery overpowered? No, certainly not. A shield warrior will still output less than any other warrior even with this bonus. So I don't consider the weapon flexibility component a problem.
Consistency-wise, it is a defensive buff which fits the idea of a shield warrior well.
Will covering vulns make vuln race warriors overpowered? No, probably not. Fire giants are probably the second best warriors (after ogres) so they are the main consideration. To take shield lore, a fire giant is sacrificing a lot because their racial synergy is in weapon types, not defenses. Shield warrior fire giant is a relatively weak choice for the cabals they can be in and the equipment they have anyways, so I don't worry about it. Feral and dwarf/duergar need all the help they can get, so I see no issue there.
Class matchup considerations:
Vs Rangers: A ranger has tactical options (archery can 50% bypass, I assume, and other paths have their advantages). As I said above, using a polearm vs mace/shield(master) is probably not usually in the interest of the warrior anyways. Very minor here.
Vs Warriors: A warrior has their own warrior lore to exploit the shield warrior not using theirs + the weapon prof advantage, so the shield warrior is at a net disadvantage, even using this new ability (outside of outmatched fired weapon dps scenario).
Vs Ninja: This will be a buff for a shield warrior vs a ninja. The primary bonus of a shield master is the domination of heavy melee combat. Generally, a warrior would never use a shield against a non-heavy melee opponent, so the primary flaw of shield lore is that it is wasted in most fights. Mya's change would mean that it is potentially useful in a ninja vs warrior fight (ninja use of fired weapons is usually a big part of their dps/strategy...moreso than warriors). While the net change isn't so great (shield warrior will take and deal less damage than a different warrior vs ninja) , it is still an advantage that warriors don't need against rogues. The question is if after the new changes, are ninja considered to be 'heavy melee' now? I can't answer that.
Verdict:
Warriors are extremely powerful already, so we must be careful not to empower them further. This change doesn't seriously empower shield warriors, but it does give them tactical options at no outstanding advantage as described above. Yet a tactical option is still a kind of buff that warriors do not need.
My opinion is to give them this buff because tactical options make the game richer for everyone. To compensate, I recommend having the shield in the fired weapons slot stop damage, but not weapon flags. Let the warrior stop the DPS, but still be potentially flame blinded/poisoned/whatever by the projectile. That will it make it a tactical option for BOTH sides, not just the shield warrior, making the game better. If the shield warrior wants to completely stop both the damage and the effect, they have to actually fully use a shield normally.
Issue: Shield lore was revamped
Verdict: Overpowered
Issue: Shield lore was twinked
Verdict: Became Underpowered/useless
It's very hard to balance it all. You made some great points @Celerity but balancing is easier said than done. You also have to try and balance the OP player vs the Newbie player vs the Moderate player.
Some classes can be played by many people and nothing seems wrong. Suddenly put that class in said player's hands and poof, it becomes OP. Then what happens is it get's adjusted to try and balance things out, then moderate player/newbie player get the short stick of the revamp.
There is a certain amount of skill/knowledge/playstyle to truly utilize a specific class, yet most people will never get to that level. So, we must always consider this element when making any adjustments or changes. I don't even know how coders do it without a test environment, kudos to you all for really trying your best to calculate it freestyle.
We are very lucky they try their best to take everything into consideration and make this game as enjoyable as it is.
How to vs understanding vs execution:
A new player will be trying to learn how to use skills and commands to play. They benefit from applied documentation (guides/strategies/builds), command consistency (melee commands lag 2 rounds, spells lag 1, etc.) and intuitive strategies (increase damroll to do more damage, stack damage to win).
An intermediate player will focus on understanding why things work as they do in order to find synergies and min-max. They benefit from transparent mechanics and lots of data (equipment database, logs, mechanic algorithms and so forth). They refine what is intuitive and apply it to the actual environment, learning the idiosyncrasies of the game (which is better? highest damage weapon or should I consider all the other variables that go into weapon choice? Wimpy, double rounds, poison usage, round counting, map layouts, exit strategies and so forth)
An advanced player will take those synergies and know when and how to apply them. An advanced player will know when to press an advantage and when to fall back. Every player may have a different level of risk they are comfortable with, but an advanced player is able to choose how risky they want to play by changing their playstyle. Therefore, an advanced player understands the role of timing commands and finds a balance between speed and safety (triggers, aliases, spamming, and so forth). This means that a highly advanced player controls the pacing of the fight. Finally, an advanced player has a great enough general understanding of how the game works that they can adapt and change their plan in a moment to most any circumstance. They can adapt with enough consistency to avoid mistakes.
If you want to make a change/mechanic that is applicable to all three tiers of players, it must be both:
Documented and otherwise accessible with an understanding of how it works
Work in a similar manner as other similar mechanics
To control the level of the command:
The more synergistic/variable the command is, the more advanced it becomes. That is because synergy requires an understanding of other/hidden mechanics/circumstances and variable probabilities means it complicates risk management (unpredictable is difficult to gauge outcomes, widens the possibilities).