Will help improving DRK's dmg and make it fun:

Bring back Scourge (with 30 second duration), make it an ability branching off of siphon strike, while not making it affected by DA.

Adding Scourge will also make you have to align it's refresh at times where you won't have DW + Delirium (40 second CD) and overcap mana by not using DA. Since it's 30 sec, you can also align it with TA, Litany, etc.

Scourge DoT ticks have a 20% chance to give you a buff that makes your next ability have the effect of DA. 15 second buff (a bit longer than DA). This gives DRK more damage and makes C&S + DA less frustrating to land without clipped GCD.

Make Dark Passenger 140 potency (the same potency that is gained from DA for most abilities) so that it makes no difference mana-wise whether you use that or DA. Can help with getting rid of mana quickly in case of overcap with DW.

Just thought of this and figured it would be worth considering. I think that sums up the concept behind my idea?