So, you have beaten the final boss and the story is completed. However, there are extra content in the game for you to continue. If you are a completionist, you'll to unlock everything, collect all items, and beat bonus bosses etc.

Let's talk about 2 things first New Game+ and Void Realm. These 2 dungeons (sort of) are completely optional, you can either try to complete one or another first. However, I'd like you complete them in order:

1. New Game+: It is recommended before Void Realm, it allows you to quickly level up to 65 which is a recommended level for the Void Realm.
2. Void Realm: This is where you grind for SP Capsule, upgrade materials, and level up further. Finally, you got to beat a bonus Yim.

When you killed Jas and watched credit, it didn't give you an option to save and reload for the New Game+, it is because you cannot start New Game+ when the story is completed. New Game+ in this game is time machine that send you back in time to start over while retaining most of your assets.

1. Reload your save game before Jas.
2. Teleport to "Shangri-La -> Altar".
3. Offer the "Tachyon Medal" you obtained from the "Sanctum - Laboratory".

Note: I highly recommend you save in another slot for the NG+ so that you do not lose your progress in the first playthrough.