ADVENTURERS! MtG cards!

Haven't you heard of Prodigal Sorcerer? and like the 5 other different variations on it?

As for the heros, shouldn't they be all partially white? So in the case of Ardam, he'll be a white-blue mage hero. Though if you are picky on their personalities/past jobs alone, here's what I would change from your picks.
Karn should definately be white not green, since green hero's are usually beasts or in some way wild, plus Karn's not exactly a nature person. At most, he can be partially blue due to his skyrian blood.
Most theifs are blue if they are without any background/group they belong too (like an elf theif or a blood theif).
HUMAN ninja's are actually blue if you look at the recent expansion(Betrayers of Kamigawa). Rat ninja's are black.
Bards are chaotic to a degree, but Gildward should just stick to white me-thinks.

Now for the villians,
I argee with bugle on Argent being black-white, and maybe also blanc. He does try to do things with "honor" and the fact that he dresses basically all white... including his hair and aura.

That's enough for now. More than enough probably.