Some of the passive buffs in the game already do have a visual effect. Some, no many.
Erebus' Vampiric Aura is an obvious one...
Sedna has a few. Divine Wind places some wisps around her, Inner Grace makes her look "pasty" and shaded out with white, and Magnificent Presence makes little white pulses come off of her.
I beleive Rook's hands and hammer begin to glow with a bloom/gold glow aura thingy when you start addding points into God Strength.
I think Oak's axe has a similar effect the more he's charged with Soul Power.
I don't believe Beasts' passives add anything, nor anyone else's that I know of.