They then say a new Tomb Raider is on the way, which suggests the big news is actually something related to that announcement, but not the announcement itself.
On Twitter, the developer has revealed that they'll be some kind of Tomb Raider event in 2018 to introduce the game, and it hints that will release shortly after the official reveal.
"Square Enix is excited to share some big news with you next year". We're getting a new Tomb Raider, that much is clear (and hopefully the PC version will come out at the same time as the console versions this time). With Rise of the Tomb Raider having been thoroughly enjoyed since its 2015 launch, fans have begun to wonder what's next for the young Croft. According to Square Enix, the first reveal will happen soon, later in 2018. We presume this will be Shadow of the Tomb Raider, though Square Enix has yet to confirm its official title. Now that we know even more is on the way, we can't wait to see what else the team has in store for us! No name, no look at gameplay, no cinematic teaser - just the knowledge that it is in development, and that a "major event" will kick off the next adventure in 2018.