Developer's Room

If you received one of the 8 endings that unlocks postgame content (any of the individual character routes, Hol Horse End or the Succession End) you'll be admitted to the Developer's Room after the credits. Here, you can talk to characters to learn more about the backstory, use the computer to make a character code for the player-vs-player minigame, and most importantly, exchange Badges of Honor for New Game + and other special features.

Talking to Steel will allow you to reset EXP levels and Friendship Points. Resetting levels will not erase skills you've learned from scrolls or boosts to base stats gained from status-up items like Starplatodine. In addition, on the fourth playthrough and up, you can save the game in exchange for a Badge of Honor.

Vending Machines
The first 4 machines from the left unlock New Game + features: The vending machines on the right unlock various equippable items and skill scrolls that can be used on repeat playthroughs. If you cleared the game 5 times, you can also buy: If you cleared the game 15 times, you can also buy: If you cleared the game 30 times, you can also buy:
 * New Game + in exchange for 1 Badge of Honor. (Free on the 3rd playthrough and up)
 * King Crimson, which gives you the option of skipping in-game cutscenes, in exchange for 3 Badges of Honor. If you already have King Crimson enabled here, you can disable it and get 10x Phantom Blood and 100,000G as compensation. (You will not get your Badges of Honor back.)
 * Star Platinum: The World, which gives Jotaro a time stop ability, in exchange for 5 Badges of Honor.
 * Josuke's Disc in exchange for 9 Badges of Honor. This is not just cosmetic, but an original story for Higashikata Josuke. However, neither of his endings allow access to the Developer's Room, so be careful not to overwrite your previous file.

Extra Boss
After clearing the game 10 times, you can battle Stroheim by talking to him.

Deus Ex Machina
If you have 99 Badges of Honor, talking to Golem (Clayman's Husband) in the Developer's Room will have him offer "Cheats" in exchange for all 99 of the badges. He warns that this will break the game, but since you have 99 badges, you've probably done everything anyways. Doing this will give you the "Deus Ex Machina" equipment. According to the description, "Prevent Ailments and Critical Hits, halves SP use." This includes every status effect except for Bashful, Corrode, Stomachache, Sick, & Hungry. It also increases evasion, and when it is equipped, it also appears as the equipment of your other party members, allowing them to gain its effects.

In addition to that, enabling 'cheats' will put 99 of every stat-raising item into your inventory, as well as 99 of all of the repackaged forms of the stronger healing items (Super Diamond C, Overdrive SP, G.E.R. Deluxe, and Ripple Champagne). Finally, the default equipment of the other party members will be added to your inventory as well.