The game will see substantial new material.
Persona 5 Royal is coming soon (at least in Japan), and the game is promising a lot of new content that was not seen in the vanilla release of Persona 5. How much new content? Well, if you have to put a number on it, roughly 30%, including new endings.
In an interview with Game Informer magazine Director Daiki Ito and Producer Kazuhisa Wada noted the amount of the new content in the game will be a way to deepen the feeling of the game and connection with the characters, similar to other re-releases the franchise have seen. It will also go past the original game’s ending, which will lead to other potential endings not originally available. They put the amount of new content to about 1/3 of the original game, making it one of the most extensive re-releases the series has ever had.