From the moment they announced there was going to be a second season of Big Little Lies, my assumption has been that this was it. The first season was to be a limited series, as the story was based on Liane Moriarty's novel of the same name. But after the enormous success of that project, there was a call for more--from fans, and likely the actors involved as well.
I'm thrilled that (for the most part) the second season was very well-received, considering Meryl was a big part of it. But the season was an afterthought, really. Moriarty wrote a 100-page novella as the template for the second season. That was then drafted into a script which gave us the seven-episode second iteration of the series. And now we have a drama series, rather than a limited series.
HBO executives have stated that the likelihood is very low that a third season will happen, which I totally expected. There have, however, been a few blurbs recently from producers/stars Nicole Kidman and Reese Witherspoon, stating that the they don't want to answer every question about what the cliffhanger from the season two finale means. "A little mystery," as Kidman suggests, is good, especially if they cook up that third season to answer those pesky curiosities.
I still think it's less than a 10% chance that it'll happen. Even if they have more to tell in the story and it gets green-lit (which shouldn't be a problem considering how popular the show is), the bigger obstacle would be getting all the starts on board again. And of course, we don't even know if Meryl's character would be a part of that third season story.
But you never know...