I think if I were to retool DRK any around the existing model, I'd want to focus on five points:
- More palpable, frequent, and varied mini-burst phases. Ideally with granular timing available, some elements of gamble, and/or polyrhythm.
- Higher APM floor over lulls. (This would likely involve increasing MP generation somewhat, forcing more out-of-burst MP spenders... which could then thereby have additional effects appended without feeling too muddied by lost bonus potency when used for said additional effects instead of solely for damage).
- Indirect throughput value to MP spending. (Think defensive components, ramping future attacks, etc.)
- Deepening a unique gimmick, such as pacing variance (e.g., via a pace-ramping Blood Weapon), or the throughput ceiling of TBN (while keeping it still balanced around mediocre usage), etc.
- Shoring up shortfalls in overall sustain (here meaning the sum of damage nullified [in excess of what would have been naturally regenerated before the next threat] and self-healing done [likewise only up to a point that would naturally regenerate before the next threat and obviously excluding what would typically be overhealing giving a --lack of-- control over its timing]).
That said, I do not think attaching Darkside costs to other among DRK's skills will make a lick of difference to DRK's apparent depth, only to its annoyingness. It'd be a resource-into-resource mechanic that's easy to master yet unintuitive and clunky to track and therefore execute upon. All floor, no ceiling, and poorly designed all around.
I'd much sooner see Darkside applicators source mechanical involvement directly, rather than simply making one set of skills dependent on two resources and thus fettering GCD attacks to oGCDs and generally reducing rotational freedom or responsiveness. For instance, imagine if each time you add duration to Darkside, you also ramped it up, increasing modifiers on both Damage (perhaps also dynamically upon certain DoT damage sources) and Haste while also quadratically increasing the drain rate of its duration so that one is forced to occasionally drop it (ideally at the very end of a burst). Think Legion Voidform for any WoW players out there. That would then allow for granular-duration burst phases and a fair degree of gamble while also offering consistent pacing variance and a much more palpable impact from resources.