[Snake]

Also could scale it with Aptitude or Will.

Scaling it with ACID ATK might be pretty awesome and finally give that stat utility, too..

Also, for the scaling. I don't 'really' know if it should be a specific element dependent, since Void is not considered an element, as Akashic is. (This is going to be the keyword, people! Akashic/Null.)

Akashic is an unresistable damage that serves as Sigrogana's 'true' damage. A flat number that will never change. And that appears on Lich's racial skill and Moonlight Mercy's 'execute'. Null is an unresistable damage with the same purpose but only aplicable to Magic Gunner's shells.

What I had in mind for Void was:
You will still use Dark ATK, but you won't damage with 'Darkness'. You will damage with 'Void' instead, which has the sick effects I've suggested.
Though, like Egil said, we could implement a math of Acid ATK + Dark ATK to result in Void damage for Black Bolt and Detogate (And god knows what awaits us in the future class updates), who knows? That one is pretty creative too.