The Unseen Presence is just an invisible aggro target for the Calphayus (similar to the Saturation Fire on Explosive Conflict or the Laser Focus on Toborro's Courtyard). It does lack the invisibility part... but it has nothing to do with a secret boss.
To be honest I doubt we'll see any more secret bosses given the resources involved in making one just for a handful of people. Hateful Entity was already a bit "recycled":
- Dreadful Entity Model (recolored blue), Dread Touch, Awakening, and AoE lighting abilities (with small variations)
- Tyrans Death Mark
- Powertech Grapple