Nintendo is releasing something a bit early for the fans of Smash in the form of the Sephiroth Challenge, a limited-time mode where you can fight Sephiroth as a boss. If you defeat him and you can use him and the Northern Cave stage prior to the release date.
Sephiroth Challenge will be live in Super Smash Bros. Ultimate between December 17 and December 22. You will need to have purchased the Sephiroth DLC / Fighters Pass Vol. 2 to access the event.