Definitely think this should be an Entropy card.
Make it work like this, and it will be more popular (My opinion.)
Dodecahedron: Spell
Cost: 4 Entropy
Ability: Target player's Mark is changed into a Quantum Mark.
Mark of Quantum: (New Mark idea)
Ability: Generate 3 Random Quanta each turn.
When this spell is used on a mark which is already a Quantum Mark, it changes it to whatever the Mark was previously.
Okay so I've just suggested a new mark card. People cannot be able to CHOOSE this mark as they do any other mark. The only way to get a Quantum Mark is to use this card. This could work in a Rainbow deck which splashed Entropy for Supernovas and possibly this card as well.
Secondly, this spell works in a way as a defense against itself.
Example:
Opponent casts Dodecahedron on my Mark of Darkness.
I know get +3 random quanta a turn, but have a Mono-Entropy deck, which splashes Darkness for Werewolves.
I happen to carry a Dodecahedron or two in my deck for just such a situation.
I cast Dodecahedron on my Quantum Mark et voilá, I know have a Mark of Darkness again.
My two cents, I think this could work.