I think Maurader/Arcanist would make a fun Dark Knight job

[Sanguisio]

DRK imo would be 30 GLA and 15 LNC, however you would initiate job quest from shanottoto or whatever her name is or E-sumi yan.
My reason being for E-sumi is DRK are based off self sacrifice to deal damage and no one else in the game afaik teaches or understands life forces etc, along with the fact why is the main healer guy in the game dressed in black with a spiked radical is beyond me lol. It would just add a nice alternate for that character.

[Katchii]

They need to either introduce a new Greatsword based class that turns into DRK or base DRK off of Marauder and have LNC and PUG be cross classed with the Marauder threat abilites being augmented to remove the threat component and add potency or a debuff or something.

[Sanguisio]

With the level cap, introduce a new job system which requires you have 3 classes levelled.
You could easily have DRK made from ACN, LNC, and GLA or a WRK : CNJ, GLA LNC

Also, could you weave in the DOH classes into this and make it interesting e.g PGL+ALC+ACN = Thief with skills based on reagents almost similar to rikku with a dampened down mix mechanic.