Once you finish the game you will unlock a new option - New Game+ - allowing you to jump into the Final Fantasy XV world once again. However, there are some significant differences between that and the regular mode.
In New Game+ you keep:
- Experience points and character levels,
- Ascension unlocks and AP,
- Survival, Photography and Finish levels,
- Team clothing,
- Royal Arms,
- Ignis' recipes,
- Regalia's upgrades and mods (except for the flying Regalia),
- Chocobo renting is available from the start.
Everything aside from the above points is cleared. You will be able to complete all of the hunts and quests again, fight powerful bosses. Additionally, you will be able to collect unique items (such as the upgradeable weapons) once again.