The Celebration

The Celebration is a future event that is common in many different timelines. In it, both Light and Dark are dancing together around giant disco ball-So they're both celebrating together. This happens after the war. The members of the Light Side speculate that it's because of a common foe-Since Sans said that Chara and Herobrine both appeared as threats, with bad things happening if they won.

Chara is good now because the corruption was removed by a Care Bear Stare, but there were timelines where this never happened-This is why Sans was able to see timelines where they destroyed the world. So the current speculation is that Herobrine is the common foe. However, nothing has been proven yet, as we're not sure whether Herobrine's potential for takeover is the threat that they're celebrating the defeat of or not.

Another possibility is that the evil villain from Sans' other vision is the common foe that they need to fight.