I love unique spell effects. Warlock green fire, glyph of the foul menagerie for DK, glyph of stars for druid, etc.
I think two key points that were well addressed here:
and here:
Are that we need to be careful about what is customizable. Because it’s important to remember that some abilities have their colors for a reason.
Some examples I can think of that could work:
Hunter traps (some of this has already happened)
Warlock Pets
Druid Forms
DK Pets
Worgen Tails
Some examples I can think of that would not work:
Mage spell color
Shadow Priest appearing holy
Hunter pet reskins
The list goes on.