So, okay, the recent letter talking about job adjustments had me curious, and... well, and I know how you're already reading this... "is this going to touch on scourge?" well, yes and no, consider how the dot for NIN works currently... I'm semi suggesting slapping it to a skill, but also adding a slight bit of RNG to it. Do I still have your attention? Well...
My suggestion is simply this... increase mp stolen through siphon strike by 50%, allow any skill that just burned a dark arts, a chance to proc a second dark arts, for free, essentially to allow the powering of another skill, this could actually justify the presence of siphon strike's dark arts version, as it could be used as a dump, that, if it also procc'ed one, could fuel a dark arts'd souleater in this way, you could also slap a combo dark arts'd bonus (you know, if you use both in tandem) to place DRK's old DoT, Scourge, which could be utilized in another way if one was capable of double dark side dark passenger for a lowered potency aoe scourge...

So here would be my hypothetical setups with this suggestion...
well consider you dark arts siphon strike, and you proc it to dark arts your souleater, it then gets a chain bonus into a bonus Scourge on top of it
or you proc it and decide you want to use it into another skill, soyou dark passenger for the aoe scourge
it doesn't really break the job, but it adds a bit of an element that can really change how the job plays
probably boosting its ability as an OT