"Every subplot gets resolved"

Zuko's mom.

I still never really got that "Where is my mother" cliffhanger for the finale. Like, were they specifically advertising the eventual comic spinoff, or did they have plans for something else that didn't get off the ground?

Even then, it was pretty unnecessary. You might not count it as "resolved" but what they showed in Zuko's flashbacks was enough for the story. But then the cliffhanger puts it in too much of a spotlight.