Let's get this out of the way first...
I got bored of this tank set up back when it was called World of Warcraft: The Burning Crusade. I'd rather we have a roster of equally-profficient tanks instead of having to pick and choose every time. You can have interesting mechanics per tank class while still aiming for the same results and performance, believe it or not.
Why would DRK ever be made into a tank? The class has historically been DPS, and is known for lesser armor scaling and greater damage potential.So i cannot agree with your new take on abilities for this class. they do not make sense with the current direction, and it makes it too similar to MRD. if anything i'd like to see it get more defensive abilities and further plant it as the go-to tank.
Design-wise, what I'm aiming for is the drain theme of DRK without going to the silly extremes FFXI's DRK went to. Enmity spikes from abilities was a bad attempt at a check/balance on DRK's higher damage potential. Notice I also suggested two ways for it to sacrifice HP (Dark Slash which is comboable and Last Resort, which is the 15-minute ability) and two ways for it to recover HP between Bloodbath and Vampiric Blade (though yes, you'd have to get creative to proc Vampiric Blade but that is the intent of the design).
I wouldn't allow a G.Sword GLA or DRK access to some of the defensive abilities. This is why I suggested splitting Aegis Boon into Defensive Focus (guaranteed block if equipping a shield, guaranteed graze if using a great sword) and having Aegis Boon be a job trait for PLD. I wouldn't want DRK getting Shield Bash, Wardrum or Phalanx. That's why I suggested more focus on spell interruption (between the G.Sword bonus to Flat Blade and the DRK ability Asphyxiate).But in the interest of discussion, if we wanted to make GLD more offensive I would give its defensive abilities a twist.
Rampart and Sentinel are tricky because they're technically survivability. I'd cap Sentinel and Rampart to maybe 20% potency baseline, and then have shield block rating/value be something of a multiplier to allow it to reach full potential. Given that I suggested DRK getting a graze mechanic to help it mitigate some damage (thus taking one step against things like "don't use melee because they die to AoE" or "this class is a pain to heal because they constantly take damage"), I would rather just have both abilities require a shield to use.