Thinking about rewrites...

[Norgarth]

Bob, you've ruined me for reading fanfic.
I got this far and immediately thought of Bugs Bunny. "You're a sorcerer? C'mon, let me see you sorcer." (Or however the quote goes.)

Who was it that wrote a novel about a "sourceror", based around the gimmick that only certain types of people were able to create new types of magic / new spells / whatev? (Thus "source"-eror.)

Terry Pratchett's Sourcery is what you're thinking of, I believe.

I believe the line went "The seventh son of a seventh son.  A wizard squared.  A Sourcerer."

Almost.  On the Discworld, they're big on eight as the mystical number, so the eighth son of an eighth son is a wizard ... and the eighth son of a wizard is a sourcerer, a source of magic rather than one who taps ambient magic.  Somewhere in the text, it's specified that preventing these is one of the main reasons wizards are encouraged to be celibate.

Ah yes, forgot that detail.