Let's wind the clocks back to 2010 when M. Night Shyalmalan infamously turned the much beloved Nickleodeon series "Avatar: The Last Airbender".
Having 20 episodes worth of content to adapt into a thematic experience, the task seemed impossible to begin with, not to mention a less-than-ideal choice of director and cast, the film was set out to be a failure.
Previous cartoon (specifically "anime" styled shows) adaptions have also proven unfruitful in terms of critical success (see Dragonball Evolution).
With this history, there would be very few out there wanting another live-action adaption of such a beloved show as The Legend of Korra. However, that doesn't mean it could not (at least hypothetically speaking) work.
Firstly there is a huge difference between the first book of the Last Airbender, and the first book of the Legend of Korra.
In the first series, we follow a trio of young characters around the established Avatar world, with new characters and plot-lines being ignited nearly every episode. And although there is the general idea of getting to the Northern Water Tribe, the audience is taken to Kyoshi Island, the Freedom Fighter's Hangout etc.
Not to mention the fact that the first series had to establish the whole mythology of the series, how the whole Avatar business works, and also provide a riveting villain story.
In the first book of the Legend of Korra, we have quite a consecutive storyline, with little distractions, fitted into 10 neat episodes. All main characters are introduced by around the third chapter (episode) mark, and the villain's back story does not appear till the near end. The storyline is more adult as well, alongside the shows characters. THe mythology of the Avatar universe is barely touched, due to the character progressions, and with all these factors, translating to this season to film seems very plausible.
But of course, the show was built on an already established universe with an already previously developed mythology. Creating a film of the first season of Korra would not be able to work.
However, what the latest (and last) series of the Avatar world managed to do, was bring in a larger audience, and get the "Avatar Universe" recognised by more people. With a much larger established fanbase, a film with a new Avatar has alot of potential.
What's more, Hollywood has finally become more open to letting people of ethnicity into a film, and although an all out cast of pure ethnic heritage would most likely not be green-lit I am sure more characters would be allowed to retain their racial origins, compared to the cast of "The Last Airbender".
And although there would be numerous amounts of front-runners for the roles, some people I think could be considered for the leads are:
(Harry Shum Jr)