I like your suggestion for a "Master Drainer LRE". Perhaps i would add something like Malefic Rapture, which deals direct damage to the target and all enemies nearby according to the amount of Shadow DoT's you have on the target. Or perhaps even just a flat increase to Drain Soul damage. Or even extending the effect of Nightfall so that you can cast Haunt or Mind Blast instantly while your Shadow Trance is active.