Following a link on the other question and further from there I stumbled over this post (http://www.theoryland.com/intvmain.php?i=428#4):
You have created some fantastic, original and well thought out
magical systems. Where did you get the inspiration for the metal-based
system of the Mistborn series and the breath-based system of
BRANDON SANDERSON: Thank you! During the early days of my career—before
I got published—I found myself naturally creating a new magic system
for each book I wrote. I'm not sure why I did this. I just found the
process too involving, too interesting, to stop.
For Mistborn, I came to the book wanting several things. I wanted a
great magic system that would enhance the graceful, martial-arts style
fights. This was going to be a series of sneaking thieves, assassins,
and night-time exploration. And so I developed the powers with a focus
on that idea. What would make the thieving crew better at what they
did? I based each power around an archetype of a thieving crew. The
Thug, the Sneak, the Fast-talker, etc.
At the same time, I wanted to enhance the 'industrial revolution' feel
of the novels through the magic system. I wanted something that felt
like an industrial-age science, something that was a good hybrid of
science and magic. I found myself drawn to Alchemy and its use of
metals, then extrapolated from that to a way to release power locked
inside of metal. Metabolism grew out of that. It felt natural. We
metabolize food for energy; letting Allomancers metabolize metal had
just the right blend of science and magic.
For Warbreaker, I was looking back a little further, shooting for a
more Renaissance-era feel. And so, I extrapolated from the early
beliefs that similarities created bonds. In other words, you could
affect an object (in this case, bring an object to life) by creating a
bond between it and yourself, letting it take on a semblance of your
Moving beyond that was the idea of color as life. When a person dies,
their color drains from them. The same happens when plants die.
Vibrant color is a sign of life itself, and so I worked with this
metaphor and the concept of Breath as life to develop the magic. In
this case, I wanted magical powers that would work better 'in'
society, meaning things that would enhance regular daily lives.
Magical servants and soldiers, animated through arcane powers, worked
better for this world than something more strictly fighting-based,
like in Mistborn.
I assume Elantris has also some inspiration, but he sadly didn't add this to his answer.
I only found this answer as a general inspiration for the book, but not the magic system itself (http://www.theoryland.com/intvmain.php?i=428#11):
As with all of my books, there wasn't one single inspiration, but a
number of them. A few of them here were: Chinese and its writing
system, and how it relates to Japanese and Korean. The difference
between teaching others of your faith in order to help them, as
opposed to teaching them in order to aggrandize yourself. What it
would be like to live in a leper colony. A king made into a beggar. A
woman who, like a friend of mine, felt she was too tall and too smart
for men to find her attractive. Magical servants that didn't look like
any I'd read about before. And the thought of telling a story about
someone who was basically a good, normal person—without a deep, dark
past or terrible hidden flaw—who got trust into the worst situation I