I kind of think Mrs. Zuko might be revealed in Book 4.
There are a few things that we’ve been kept in the dark about regarding the gaang and I can’t help but think its because they’re all related.
1. Zuko’s family. Who did he marry? What is their daughter like?
2. Sokka. Everything about Sokka.
3. Who fathered the Beifongs.
My personal theory is that we’ll get a glimpse into the Fire Nation next season and come to find that the Firelord’s mother was a Kyoshi Warrior and not Mai. Which would really send the fandom into a frenzy, because that would take Suki out of the Tokka equation. So, I think some part of that will be revealed in Book 4.
Maybe who fathered the Beifongs is unrelated, but I can’t help but be suspicious that we know nothing of Toph and Sokka’s personal lives and were left off in the comics watching Suki and Zuko take a special interest in one another after he’s broken things off with Mai. Its all pretty suspect.