Sanctuary

The Sanctuary, where you can talk to the ghosts of bosses you've defeated, is accessible through the left door of Emporio's Room. Fans of the SaGa series will recognize it as the final boss room from the very first SaGa/Final Fantasy Legend game, which the spriting style of 7th Stand User is based off of. Besides chatting with the ghosts, there are a couple extra events and boss fights here that you can access once your level is high enough.

You'll also be booted to this room if you fail the final confrontation with Vins. You will be unable to leave the room, but can talk with any of your party members that died, see some different dialogue from Dio's followers (N'Dour, Kate, and Amber, for example), and return to the title screen using the door to the afterlife on the right side.

The Orb
Near the bottom-right corner of the map, there's an orb. If Abdul, Kakyoin, Joseph, or Polnareff's level is 40 or above, you can bring them here for a boss fight. Winning the fight will grant you +3 FP with that party member and a Badge of Honor. (Once Abdul, Kakyoin, and Polnareff have triggered this event, the other two will be barred from it until the next playthrough, but Joseph's event can be triggered regardless.)

Interacting with the Orb after losing the final battle against Vins will show an additional scene. This event will not occur with any Zeppeli Family events or Dissonance.

The God-Slaying Axe
After defeating Alessi, talk to him, and he may reveal the location of his axe. Check a certain spot in the river to start a boss fight by choosing the middle dialogue option. Winning will give you an item that will make the following battle with the Creator substantially easier. If Joseph is in your party, the dialogue prior to the fight changes.

The Creator
If Jotaro's level is above 40, talking to the man who shares Speedwagon's sprite will start a battle. Defeating him will cause him to disappear, allowing access to the door behind, which leads to the Debug Room. Note that he will respawn if you leave the room.

(Note: The dialogue in this battle is a reference to the final battle of The Final Fantasy Legend. There is a glitch in that game that allows the Creator to be insta-killed with a Chainsaw, which is why the item from the event above can be used to defeat him, and also why he will bar you from using Joykiller or Carpenters' Chainsaw attack. The Creator's sprite is ripped directly from the game, and despite the striking similarities, he has nothing to do with Speedwagon.)