I think you highlight the issues well. Also, funny that you mention about delirium providing an effect. I was just thinking that one way DRK could start changing its tune is for one of its combos to have a movespeed drain ability. For example, +10% for DRK and -10% for the target. I think it'd provide DRK not only a solid personal buff but also pave a way as an option of a more of an indirect counter to burst, by eating a player's MS (even though -10% is light).

Whether that idea in a vacuum by itself is good or not, though, isn't a big deal. But the grander idea that souleater/delirium, and even power slash might be able to have some absorption type abilities would go a LONG, LONG way to making the class a lot more viable by providing options that are either direct or indirect counters to overwhelming burst. A DRK with a delirium that drains movespeed, souleater that drains stats, and a power slash that steals single buffs would be an example of something they could do that can provide those counters. And if they start adding these types of mechanics in, the game mode itself will become more diverse, complex, and enjoyable overall.