Well not to rain on everyone's parade but you guys do realize that Nick's record with big budget animated movies is pretty mediocre as compared to their more established rival with the Mouse. Rugrats, Wild Thornberries, Spongebob...these were shows that appealed to ages 6 and up and the reason why they did so well is essentially because for every kid screaming to watch the movie, mommy and/or daddy need to pay for a ticket too.
Legend of Korra is to me is pretty damn niche. Unless they are trying to appeal to the Teen Book reader crowd like Twilight fans or Hunger Games fans who are for the most part....female...they are going to have a hard time selling it without a couple of really marketable faces as guest stars. Not to say the movie if ever made would flop but for they budget they plan to put in, they might not get the returns they desire.
Also in the tradition of animated movies based on a TV series i.e. in Anime, I'm never sure how well they do. Look at my Avvie. Do you think Awakening of the Trailblazer did well? Sadly no.