Zuko, Katara, and Lin Beifong vs Mako, Korra, and Bolin.
(In their current state. I bet even though Zuko and Katara are older they can kick ass)
1. What happened to Zuko’s mom?
It’s apparent at the end of A:TLA, that Zuko’s mom is still alive, and that Ozai seemed to have banished her somewhere where she could not get in contact with Zuko. Also, at the beginning of A:LOK, Katara is asked that question right off the bat by Jinora, which Meelo interrupts, so Katara never explains. I hope they tell us soon.
2. What became of Azula?
Fire Lord Ozai, no longer considered a threat with his fire bending taken away, was locked up in prison in the Fire Nation. But what about Azula? The last scene with her, she was going mad after Katara defeated her during her Agni Kai with Zuko. Did she suffer the same fate as Ozai?
3. Did Toph ever get to see her parents again?
After Toph ran away, it was obvious that she missed her home and wanted to communicate with her parents. She even had Katara help write a letter to her parents. And Toph fell into a trap when she got a false letter claiming her mother was in the city. Did she ever return?
4. King Bumi?
We know he took back his city during the Solar Eclipse, but at his old age, for how much longer did he rule the city? Although the Fire Lord was defeated by the end of the series, we can imagine that pockets of Fire Nation resistance still existed throughout the Fire Nation and the world in general. So has Omashu remained safe?
5. Long Feng and the Earth King?
After Azula took over Ba Sing Se, it was implied that Long Feng was imprisoned. But after she was defeated and the city taken back by the Order of the White Lotus, what became of Long Feng AND the Earth King? At the beginning of Book 3: Fire, we know the Earth King and Bosco left to go explore the world, did he ever return to reclaim his thrown? And was Long Feng forever banished from Earth Kingdom Politics? Were the Dai Lee dissolved?