According to co-creator Damon Lindelof, you shouldn't expect everything to be explained in the final season of ABC's Lost. In fact, some mysterious characters may be missing entirely from the last set of episodes. Spoilers!
Talking to Comic Book Resources during an appearance at LA's Earth-2 Comics, Lindelof said,
I don't think it would be 'Lost' if we answered every question to every viewer's satisfaction. I mean, there are some people who are still asking us 'What's the story with Kate's toy plane?' and there is nothing more to say about it; we've definitively answered that question to the best of our ability on the show, so you won't be hearing anymore about the toy plane. That being said, there are other sort of meta questions, like, what do the numbers mean, that we will be addressing more directly in the final season. But some people will feel like, 'Wow, they answered more than I thought they ever would about that question,' and some people will say 'What a hose job, I am so unsatisfied!' Our goal is to land in the middle of the 'hose jobers' and the 'too much informationers,' because you can't make everybody happy... Most of the questions that we get asked [now] are 'Is so-and-so coming back?' or 'Are we going to see more of so-and-so?' and I feel like that if I know the answer to that question, the fans sort of deserve a 'Yes, that is something you should be looking forward to,' or 'Don't get your hopes up for that because it's not going to happen.' I don't want you to tune in waiting for the Great American Libby story, because it's not coming.
Libby's not the only character who'll remain a mystery, it seems; we've already told you that Michael won't be appearing in the final season despite rumors to the contrary, and now Entertainment Weekly's Michael Ausiello is saying that Rosseau won't make an appearance in the final episodes, either, due to a scheduling issues for Mira Furlan, the actress who plays the show's favorite crazy French woman. We could live without Malcolm and Libby, but no Rosseau? That could be a dealbreaker.
Damon Lindelof Is Still "Lost" [Comic Book Resources]