I think that the addition of any GCD would be of benefit, given that DRK has 4 GCDs in single target and 3 in AoE. My recommendation would be to have something combo off of Bloodspiller, because not only would that break-up the base rotation, but it would also turn Delirium into a 6 hit sequence (which could have a speed buff attached). There are other solutions as well, but you have to keep in mind that expansions typically only add three actions at a time, so you don't want to expend too many of them trying to build up a new combo sequence.