We'll find out about Zuko's mom next month with the second The Promise graphic novel, shouldn't we?
I think you're severely underestimating the ability of fans to complain.
Life is like a roller coaster. It has its ups and downs, but if you sit back and relax you get a heck of a ride.
Wired Magazine, on Korra:
And this just in: The show was Nick's most watched animated premiere in three years. Take that, idea that girl led shows won't draw viewers.