Another Zuko adoption thought: Zuko adopted by Spirits? Hard to say if Wan Shi Tong would kill him the moment he stepped foot in the library or not. (He remembers Zhao's visit all too well.)
I totes ship Wan Shi Tong as a surly Zuko parental figure. We'll get a little bit of that in Dark Night in Ba Sing Se (because what is a Ba Sing Se fic without Zuko getting into spirit troubles? Embers started a proud tradition and I would be a fool to break it), but no formal adoption. Foxfire by Rahar_Moonfire comes closest to this.
The "Jun and Hakoda co-parent Zuko" fic will also have some Wan Shi Tong, but he's more like the dickish uncle who always puts inappropriate gifts in the Secret Santa than a father figure. "He Who Knows Ten Thousand Ways To Be A Smartass" is one of the first Zuko lines I ever wrote for that outline. (And Jun's response: "How many have you learned?") ("Only a hundred.") (And then there are hugs and the Gaang is like "holy shit Zuko's mom is HOT." Or maybe that's just Sokka, it's not like he's comparing Zuko's-hot-mom notes with the rest of the Gaang.)