As far as DRK utility, it doesn't even need new or different abilities, it could work with added Dark Arts effects.

Dark Arts + Salted Earth:
Adds vulnerability to damage over time effects. DoTs deal 10% more damage to any targets within the Salted Earth area. This is a unique effect that doesn't conflict with any other jobs, but does provide support to the group.

Dark Arts + Blood Weapon:
Weapon skills add a debuff to the target lasting 4 seconds that causes any hits from anyone in the party to drain a small amount of health. The debuff is refreshed each time a weaponskill is used while under the effect of Blood Weapon. This would be a small aid that could be used in situations where there is consistent raid-wide damage to help take a little pressure off of the healers (think Drain Samba for anyone familiar with FFXI).

DRK doesn't need a buff, but some added utility would be nice. These are just a couple of ideas that I had.