I'd be fine with them just buffing Souleater to have extra health return similar to how Inner Beast worked circa 2.0 Final Fantasy XIV WAR.

As of right now, Inner Beast on 3.0 WAR heals more and gives 20% mitigation for 6 seconds, and it's not like FFXIV Gil Souleater can be used much more frequently than Inner Beast to justify this disparity.

I feel that Final Fantasy XIV DRK self healing is suppose to make up for their lack of physical mitigation, but it's really not there. Instead of just giving them a big burst heal alone it would be nice for them to add healing effects to other abilities, like have Shadow Wall have some sort of regen effect.

I'm personnally against such a heal, as a dark knight should always be about drain from the ennemy and not self abilities.

The problem is that dark arts is way too expensive to be efficient. It's simply not worth it :

Souleater : potency of 260 without dark arts, 400 with it. would you use all that MP for a 140 potency strike+heal ? I don't
Dark Passenger : Final Fantasy XIV Dark arts is mostly here for the 15s blind ... which most ennemies resist.
Dark Mind : the 15% extra magic resist could be useful, very situationnal though.
Abyssal Drain : Yes, the heal is good if you get at least 4 or 5 ennemies around, but again, not that much (you should have 10 mobs around to get the heal equivalent to Equilibrium)
Carve and Spit : 450 potency is great on paper. But we are tanks.

That's the whole dark arts mechanic that needs to be changed. Either by halving its cost Cheap FFXIV Gil to be able to use it more often, or by changing its effects so that you actually want to use it.