I'd probably do something along the lines of:

Holmgang - 6s duration - Can't go below 1HP - Additional Effect: Forces Enemy to attack you for the duration (Basically like a Taunt from other MMO's) - 240s CD

Living Dead - 10s duration - Can't go below 1 HP. Damage that would otherwise cause you to go below 1HP instead creates a shield that absorbs healing for the amount of damage prevented. If this shield remains at the end of the duration, you die. - Additional Effect: Increases healing received by 30% - 300s CD

Superbolide - 8s duration - Cannot take damage. Reduced to 1HP. Additional Effect: Refreshes the CD of Aurora - 300s CD

Hallowed Ground - 10s duration - Cannot take damage - 360s CD

Basically, streamline the CD's so they're closer to each other and put some additional effects onto some. Now the biggest difference in CD is 2 minutes (Between WAR and PLD) but the rest are +/- 60s so they're more on par.

Warrior gets some extra utility in that instead of having mostly redundant Binds, that they instead get forced taunt for the duration which can give them some utility to "Tank Swap" just to soak a TB with Holm. It also fits the theme of "Holmgang" better (Which is a ancient Norse duel). It gets a lower CD than other TB's at the cost of lower duration.

Dark Knight basically gets Purgatory from WoW DK's. Where instead of needing to be healed 100% of their max health (Therefore being penalized for having higher HP pool) they instead only need to be healed for the overkill damage, this allows them to do things like use TBN to help minimize the amount of overkill damage as well as improving as their health pool increases and so the amount of overkill damage from an attack decreases. To add a little extra to the mix, give them increased healing taken so that even less healing is necessary to save them, also without this dispelling the skill itself, meaning they can get full 10s (Well, more like 9s) of hits that bring them below 1HP so long as they get enough healing.

Gunbreaker gets not a ridiculous CD for being literally worse Hallowed Ground. Also, some extra self synergy with the Aurora refresh, allowing them to use the skill even if it was on CD from a prior use. Thus allowing them to tick up their own HP over the duration.

Hallowed Ground just gets a reduced CD to fall more in line with other invulns. No other additions are necessary as it's already very strong as is.