Whenever I select my second answer, it clears my first answer.
Yeah, I don't know how to answer this. Lack of mobility is a problem because of the lack of healing, which results in a lack of tankiness. Lack of damage and healing and tankiness and mobilty make sustain an issue because you can't 1. Burst people down 2. Heal efficently 3. Disengage to get resources back / mitigate some damage. This forces you to burn resources and cause a problem for sustain.
We all know there's no single answer. This poll oversimplifies multiple complex and interconnected issues with the class.
My perspective comes from discussions with mDK players and from playing against the class. The issues as I see them always distill down to needing more reliable healing and unhinging burst from expensive ults.
Perhaps a larger DBlood heal would be sufficient, but consider that mDK's only heal over time is Rapid Regen/Mutagen. That is a hugely underappreciated weakness.
And they are often forced to ult for resources, leaving them lacking burst when a kill presents itself. An execute is not necessarily required, but it would help. Regardless, increased non-ult burst or a less expensive burst ult would provide a huge benefit to the class.
Sustain is strained for mDK in part because they have to spam wimpy heals and noodle whips to do anything useful. Perhaps they do need some sustain love too, but I'd like to see functional buffs to their healing and kill potential first. They need those changes for sure, and I hesitate to give anyone too many buffs at once.