I know that having character's appearance change when they have different items equipped would be very difficult. for example, QofT would look a little strange with just the addition of boots, or Rook may not quite fit into a suit of armor. But what about having an Almighty look for each of the Demigods. Once they reach level 20, they get a new, more powerful look. For example, Erebus would get a helmet and a larger mace, rook would be 10% larger still and have spikes on his knuckles, QofT would be in a black lotus flower with even more vines protruding, UB would leave green foot prints wherever he walked... Something to show everyone you have maxed out your character and they are all powerful. It wouldn't necessarily have any game affect, but would just look cool.
love it! and for the rook, I wish he had two energy towers on those stands on his back, and the shoulders would be arrow, and once he is at level 20, he could have tiny little towers all over him, and the spikes on his knuckles like you said, and a bigger different hammer.
UB could also have a toxic green smog emminating from him
Regulous could get six wings like the seraphim
Demon assasin could get two more heads and have seven tails
Oak could get a bunch of glowing shield pieces orbiting him
BUT i believe that this is only a sweet dream...