The first castings have been officially announced:
Maisie Williams as Wolfsbane
Anya Taylor-Joy as Magik
... which were the castings debunked last year.
http://www.hollywoodreporter.com/hea...lliams-1002743
With Cannonball, Moonstar, and Sunspot announced as the other roles it's a fair bet that they're aiming for the early Claremont era (Warlock is also supposed to be in it), and the main antagonist is Demon Bear.
The original Demon Bear run was issues 18-20. It's collected in https://www.amazon.com/New-Mutants-X...XGS/ref=sr_1_1, and they're available in Marvel Unlimited if you're a subscriber.
Scheduled release date is April 13th, 2018