The duration of an invuln really isn't all that important in most cases. I remember Holmgang having a 6 second duration with a 3 minute recast and it was even more versatile than it is now. Standardizing everything to 10 seconds just took away the argument that a 2 second duration difference was the only reason why the recast advantage exists.

Yoshi-p confirmed in a recent Korean Fanfest Q+A that there will be a new deep dungeon in the 6.x patch series, as well as the possible of more Bozja style duels, which has me fairly excited about the solo content that's going to come out of it. It would be nice if DRK had an invuln that provided a benefit even in the absence of an external healing source. Holmgang and Superbolide work fine on their respective tanks because they both have burst healing options. I think that even if you put Superbolide on DRK as it is now, you'd struggle with it in solo content.

The current iteration of DRK is very much about anticipating damage and not letting your HP get critically low due to a lack of recovery options. I think that any redesign of Living Dead should take this into account.