The issue I have with a third combo finisher on DRK is that I just don't know what it would actually do. If it's a buff/debuff/DoT that you maintain, then you're just cloning Goring Blade/Storm's Eye, except in a way that's disadvantageous to DRK since Bloodspiller makes DRK's GCD rotation less predictable than WAR or PLD. If it's just more damage, then either Souleater or the new finisher is stronger, and you just always use whichever has more effective potency.

I think there's more of an argument to be made for extending the combo to four GCDs, like DRG, but I don't know if I really like that idea either. Personally I want to see more Blood-based, non-combo GCDs to break up the core Souleater rotation than an actual alternative combo.