I don't know, Thoth. There have been absolutely no casting announcements at all for them. Every other character? Yes. Some fans are speculating that they're going to use Osha to take over the Jojen's role(s). They guide and help Bran, so maybe they think Osha can do that instead?
I still hope we see them, maybe in season 3, but by the time.... (spoilers - highlight to read) >>>>> Theon takes over Winterfell, Bran, Hodor, Osha and the Jojens are hiding and the Jojens seem to play an important role in keeping Bran going<<<<
(big shrug)
They haven't announced any of Cat's brothers either from what I know. Can't remember if they mention her father at all.
Some characters I think can go. Many of the Iron Isle folks except for Asha and Victarion (and Theon's father). I think Victarion will play a larger role in the next book. The people Arya runs into in book 2 and 3? So many of them are so unnecessary. Of course we'll get Ja'qen, though. A man thinks that's a good idea.
They'll get into trouble when they have to start introducing the Dorns and the Tyrells. I mean, the Red Viper is imperative to season 3 unless they give that "honor" to someone else during you-know-who's defense.