Almost all Avatar fans are hoping for a season 4. Although Mike and Bryan (Avatar creators) have told that they aren't planing one, I can't see how or why they wouldn't.
The KCA (Nickleodeon's Kids Choice Awards) showed that Avatar was voted the best cartoon on TV (out of listed shows) so by not making new episodes Nickleodeon is realy losing money they could get from making new episodes. They show also left a lot of unanswered questions.
They only other option I see feasible is that they show answer these questons in the live action movie. But I don't see how they'd do that. They're unquestionally going to take a lot of things out in the movie, so why add more things that weren't in the show in the first place?