Since there really isn't any interest in my plan, I'll change my vote. Ultimately I think having something different than straight canon is nice, though I'm not sure if I'm keener towards scenario or character as the divergence.
The benefits of a canon character in a new scenario are we have a sort of basis of the character to work off of and we get to see how the character has changed due to the scenario. Zuko's almost certainly not burned... and Iroh may even be Firelord.
The benefits of a new character in the canon scenario are fairly obvious we know how things vaguely work but we get to directly be the new cog that changes things maybe we change up one of the main group, maybe we become a new member, maybe we are completely separate doing something on our own in this massive war. These are interesting certainly.
However, as I write out these thoughts I become a bit concerned with the idea that we'll end up using foreknowledge in the canon scenario; now such is easily enough ignored and not given opportunity to provide actionable merit, but its certainly there and even inadvertently hard to ignore, so I think I prefer a new scenario ultimately.
[X] Plan Flow of Destiny