Warning: Braindump
Maybe a bit off the beaten path, but what about some variant of the following as a replacement for Parry (taking a page from LoL)?
Tenacity: Reduces the duration of (some?) negative status effects by a percentage, based on the amount of Tenacity you have.
I imagine scaling would have to be rather gradual, and likely max out at a certain amount (15-20%?).
Devs would have to be very careful with the tuning of such a stat, but it would be interesting as a tank to have a stat that might - for example - decrease the time concussed in A5S by 1-2s, shorten weakness duration further for tanks (beyond the FC action).
Might be confusing though, if devs decide certain debuffs / abilities are immune to reduction via Tenacity, for example the elemental debuffs in M2S (or even T9) - probably wouldn't want those going off sooner than normal on tanks.
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Hmm, another approach that would be easier to work with is to give tanks a secondary stat that reduces their cooldown recast durations (like the AST's Spear card). Useful in tank stance (e.g. can shaving off 2s from Rampart or 5-10s from Sentinel), and still useful (albeit less so) as OT (more Berserk, Blood Weapon, Fight or Flight uptime).
Not sure what to call this secondary though. Vigilance?
My take would be that through intense training (represented by this stat), tanks would be more able to push their own limits.
My main concern here is whether this stat fits with the devs' design philosophy for tanks. I mean, it's interesting how AST can break this mold, but they have to wrestle with RNG to do so.
I would personally love to see a stat like this, since it would allow you to customize your rotations based on gear, similar to how Skill Speed can affect your GCD rotation.
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Originally I was thinking of something that reduced weakness duration, and base HP recovery/tic, but that seemed rather weak still, even though it's applicable regardless if you're MT or OT.
Some other routes might include replacing Parry with lifesteal for tanks, with overall damage reduction - could even be split into multiple secondaries for Phys Dmg Reduction and Magic Dmg Reduction, maybe even some sort of tank secondary that gives a small defensive boost to nearby allies (auras - more of an ability / add-on to tank or dps stances, but maybe there's a possibility of working in a new defensive secondary stat).