I wonder how the side-characters are going to be, if they even exist...
And already knowing 3/4 elements? I can't see this going past the first season, 2 seasons may be doable, but 3 episodes seems insane. That is assuming that there isn't some kind of plot twist that could elongate the series. Like each nation having their own mini "Fire Lord" that she has to beat or something.
Actually having her already know three bending styles was a good choice in my opinion. If we had her going from nation to nation trying to find teachers to teach her each bending style it would basically be a knock-off of the original series and would be uninteresting. Plus it sounds like the story is starting the second she enters that specific city. Besides, the main conflict happens in the city so having to go through all that again would be unneeded.
Toph was pretty young in the series too, being 80-ish, there is still a chance she can still be alive, or even sokka or katara. I doubt they'd be main characters though, probably just cameos and thats it.
Katara has the highest chance of getting a cameo being Korra's teacher's mother, then Sokka,