Kathryn Newton, who played Madeline’s daughter Abigail in Big Little Lies, briefly mentioned the possibility of a season 2 whilst promoting her new movie Lady Bird:

Is there anything you can tell me about Big Little Lies season two?
I don’t know what I’m allowed to tell you. All I can say is that after the Emmys everybody was very, very positive. I can’t say anything. They all just looked at me funny, with a little wink and a smile, whatever that means.

(Vulture)