The last mod slot is left open intentionally for the Augment for Nova's 1, Null Star/[Neutron Star].
This skill gains more damage reduction per orb, which scales off of duration. With the current small bubble setup, you would have x18 (Max) orbs for a damage reduction value of 90% (5% damage reduction per orb, x18 orbs with this setup), as well as the ability to refresh the skill with the augment.
With arcane [Guardian] proc'd, and maximum orb value from Null Star, we have an effective HP pool of 35,680. Making that 356,800 EHP when accounting for a fully stacked [Adaptation] as well.
I've dreamed of making [Limbo] as tanky as possible, and the new helmnith system might just make this come true.