I disagree with opening with a Souleater combo, as the people I run with will often start unloading their max DPS in their openers and not having that Power Slash could spell disaster.
Very often I find that doing Unmend + Plunge results in the leap from Plunge happening but the skill not actually going off. I don't know what the issue is. But if it's currently unreliable like that then I'd say don't use them in tandem.
I do think that Dark Arts should be popped first before engaging, because even if you only get one out of combat MP regen tick that still helps, and running out of time to use up Dark Arts isn't an issue since Carve and Spit is used so early.
So ideally my opener would be:
Dark Arts -> Plunge -> Scourge -> Low Blow -> Hard Slash -> Carve and Spit -> Spinning Slash -> Dark Arts -> Power Slash -> Salted Earth
Now a couple questions:
Dark Passenger. When should I use this? Should I incorporate it into my oGCD rotation only when I have high MP? Should I only use on groups? It seems like a skill I would NEVER want to augment, since as an MT it reduces parry procs and Blood Price hits and as an OT that Dark Arts MP should be used on better things.
Carve and Spit. I haven't reached 60 yet (damn dayjob) so I haven't played with it, but to me it seems like for main tanking it would almost be better to not use it with Dark Arts outside of the opener and rather save it until you need emergency MP? It does okay damage for being oGCD but on a 60 sec cooldown it would really suck to be out of MP for whatever reason and lamenting that you squandered an MP regen move in order to push that deeps brah. I dunno, maybe as people get more comfortable with MP management and start to really know fights and who they're running with, etc, then sure go ahead and incorporate it into your oGCD rotation, but I feel like in content pushing/PUG scenarios it might be better served as an emergency fallback? Obviously augmenting it would be something you'd probably only want to do as an OT, since augmented Souleater is probably more useful for MTs.