Five years ago, Sega announced that it would be developing a "super game" that would be the biggest project they had ever undertaken. We never received any details, but the idea was that it would be one of the many live-service projects announced during that era, and Microsoft was also involved to some extent.
However, silence is often a worrying sign in this industry, and the lack of leaks from major projects usually indicates that things aren't progressing as planned. And sure enough. In its latest quarterly report, Sega now announces, quite matter-of-factly and without further explanation or justification: "Decided to cancel Super Game. No additional costs associated with the cancellation."
Otherwise, no other titles appear to have been affected, and Sega specifies that Virtua Fighter 6, the new Crazy Taxi, the Jet Set Radio reboot, a new Streets of Rage, and the sequel to Alien: Isolation are all still in development, even if they don't want to announce any release dates yet.
When major games are cancelled, information and assets usually leak out in various places, but this approach is a bit more unusual for Japanese developers, so there's a risk we'll never learn anything more about the game (such as whether it was based on a classic Sega brand, genre, game type, and so on).