A user on Reddit posted a picture taken on the subway that shows an individual’s laptop that is seemingly working on Shadow of the Tomb Raider. This title seems likely as Square Enix seems to be transitioning to the “of the Tomb Raider” suffix in its titles.
This also doesn't come as a surprise considering the critical reception of the previous two titles. It isn't uncommon for a game’s title or existence to have been leaked because of developer worked on assets in public. Kotaku went on to confirm with their sources that this title is indeed the title of the next installment in the series.