From what I remember of Dark Knights in XI, they primarily focused on 'drains' and 'absorbs', as well as the use of minor black magic. There would need to be changes to their casting in order to make them viable tanks and still have these abilities, however. It'd be useless to have black magic and not be able to cast due to interruption from damage.

The core idea for a revised XIV DRK tank would be a persistent buff that would give them a sort of 'bloodbath' lifesteal effect and would then be enhanced upon by other abilities in their repertoire. In addition to this, have their combos not only increase enmity, but absorb attributes from their targets to enhance their own. Either this or have abilities to take care of this and make the combos deliver elemental damage depending on how they're executed.

Of possible additions to the tanking role, I think it would fit perfectly in its own way.