Are you suggesting have the balance prism convert into shadow damage? That would be VERY problematic for the game as a whole.
IF balance is going to get a prism, which it definitely needs, it should either: Get 2 prisms, one that goes elemental and one that goes spirit, OR Have the prism do damage of a RANDOM school.
I'm not sold that a prism will fix balance's problems, but giving balance essentially Shadow Judgement is VERY bad for the game without more shadow resist. At which point Darkmoor becomes a joke.
Also there could be spirit and elemental prisms like spirit traps and elemental traps. There could also be blade versions of prisms but that is arguable. Another idea for a balance prism is that the damage doesn't really need to convert to a specific school, there could simply just be a boost applied to the damage. Does it matter if the damage converts to a specific school for the boost?