challenging for block-capped tanks it's impossible
So you've basically just said that DKs will need to stack Stamina at the expense of everything else in a world where block-capping occurs.
In order for a DK to be able maintain their survivability, they have to be able to survive the incoming damage first. If you intend to make the incoming damage challenging for block-capped tanks it's impossible.
Block capping presents two paths if it's not seen as a problem:
1) Block tanks take so little damage that they wow gold are by far the preferred tank for anything.
2) In order for block tanks to have anything resembling threatening incoming damage, DKs get 2-shotted.
I think there's a disconnect somewhere.