DRK needs a revision

[Reynhart]

simply adding a gauge similar to the Warriors Inner Beast gauge

Yeah, the gauge feel a little shoehorned...

First, it's the only tank gauge that's not tied to an existing skill, and thus, above lvl 60. And second, it's doesn't really fit into DRK's lore.

What could have been interesting, IMO, would have been a Darkness Gauge, obtained with Darkside. This gauge would have filled overtime and only be reduced by using Dark Skills (Dark Arts, Dark Passenger, etc...).
The higher the gauge, the more powerful your skills are, but let it fill completely and you'll receive the Down for the Count debuff for a few seconds (Like the DRK lvl30 quest) emptying the gauge and canceling Darkside.

The trick would be to keep the Gauge at a certain level and managing your MP so that you always have a way to reduce your Darkness. And each Dark Skills would have a different "cost" on the Darkness gauge.