Not much is known about the story at this point, except that Samuel L. Jackson confirmed his character of Frozone will be in it.
Here’s the Tweet Disney/Pixar sent out earlier with this date, along with their other upcoming projects:
We’re proud to present the official release dates for our upcoming slate of films through 2019. pic.twitter.com/1cbmRnVv03
— Disney•Pixar (@DisneyPixar) October 8, 2015