We can probably guess the spells since we know they're going to be DRK specific and Dark Magic Based, as Dark Magic falls into several catagories: Absorbs, Vampirism, Enfeebling, and, the newest branch, Enhancing.
There aren't many things left for us to steal with absorbs so if there are new absorb line spells:
Absorb-Speed: Increases attack speed of caster and reduces the attack speed of the target.
Absorb-Enhancement: Steals one of the target's beneficial staus effects.
We've pretty much got the basics of Vampirism with Drain and Aspir, so if they added Vampiric like spells:
Aspir II: The most obviously expected spell.
Of the "Enfeebling" category, while absorbs fit into this technically, we pretty much have just Bios. If there's a new spell it probably won't be of Bio line (Unless bioga orz):
Defensive break: Lowers target's defense.
Terror: Obvious what it does, but doubt this would be implemented.
Enhancing is the newest category of Dark Magic(Absorbs technically fit here too), with its only representative being Dread Spikes. If there are new spells in this line:
Endark/Endrain: Really obvious/possible choices for DRK. Fits the DRK playstyle.
Dread Spikes II: Possibly an additional spell. May have longer duration and higher hp%, used with the original to shore up the problems with it. Unlikely though.
Dark Spikes: Deal Darkness Damage, it's edge over dread spikes is that it would cost about as much as normal spike spells, last as longer, and shorter recast. Unlikely, but sure as **** more likely than Dread Spikes II.
Aside from that, they could add a line of DRK only Dark Magic nukes.
I'm not saying that the **** I listed will all be used, but it they're giving DRK new Dark Magic, the spells will probably look something like one of those things listed.