You’ve finally defeated the last boss and get to sit back and watch the ending, although you get a choice of what to do. This affects the ending you will view, but not much else. Eventually, the credits will play and there will be a scene after the credits are over. Once you reach the “Fin” screen, wait for a few moments and you will get some unlocks. The first thing that unlocks is a new artifact, the Pengy artifact. This basically makes it so you can explore areas without having to do battles, but you should have explored most of them already.
Make sure you save your game to a new file when prompted, just in case. The game will spit you out back on the Moon, so you can refight the last bosses if you wish, or explore the world and do the rest of the optional content. The other thing that unlocks is a new NPC in Fade Ridge in the Fade Realm, where you can rebattle any of the storyline bosses, but they will be at your level, so they will always present a challenge. Also, the NPC will offer a few new optional bosses that you can’t fight anywhere else.
Outside of the above, you have the Ravine of Memories, where you can challenge three new bosses. The normal enemies in the Ravine of Memories can drop new weapons, as well as weapons with additional effects, which you can transfer over to your other weapons at any blacksmith. To make things extra dangerous, there are two Mecha Pengys in the wild, one in Ancient Ruins Merhl (where you fought the boss) and the other in Ragness Ruins (where the vulcosuits were). These enemies are very tough and drop keys that allow you to open up locked spheres (Pale Tower in the Fade Realm, and Dhaka Venu in the Fade Realm). These will grant you two more vulcosuit point upgrades, making it max out at 400.