You're making my inner Inigo Montoya twitch. "Mary Sue" does not mean what you think it means. The problem with Mary Sues is not that they're cliches, it's that they're poorly thought out, generally have little if any basis in the actual setting (because they tend to break basic rules about the universe), and are built solely to attention whore, logic or plausibility be damned. They don't want to RP, they want to be special snowflakes.

The road to Mary Sue inherently begins with trying too hard to make your character "unique", mostly at the expense of common sense or rationality. Let's just approach this logically, why in the hell is Illidan and Onyxia's child hanging out at the Wayfarer's Rest in SMC regaling any passerby who stops to look into her achingly beautiful "luminescent orbs" (that change color with her mood) with her lifestory of how Kael broke her heart and that's why she cuts herself (to feel again, of course?)? Setting aside the blatant stupid it doesn't even make any damn sense.

Mary Sues don't "spice" a damn thing up, they contribute to a toxic atmosphere of selfishness that ruins everyone else's RP experience. The best thing to do is ignore them, or as my catty mage is wont to do, mock the shit out of them when they start carrying on.