Beyond a solitary tweet and some accompanying artwork there's nothing to go on yet, however the image below portrays what could be a meteor breaking through a planet's atmosphere, or perhaps a plane or spaceship crashing from above. The lone figure looks to be dragging a tattered parachute which has seen better days, and also appears to be wearing a space helmet and jumpsuit.
Thanks for your warm reception of INSIDE. Since release, Playdead founder Arnt Jensen and the team have been working on the next adventure. pic.twitter.com/RfejnH39mRJanuary 20, 2017
As for what Playdead is working on next—your guess is as good as mine. Take from the above image what you will, and please do share your own suggestions in the comments below.