Tie-Ins To All Other Persona Games
Initially, Persona 3 and Persona 4 seemed to have almost nothing to do with each other outside of being of the same name, and both worlds colliding at a small field trip in 3. However, in the downtime between Persona 4 and Persona 5, Atlus has been knitting together the tales of Persona 3 and Persona 4 with the Persona 4 Arena games and Persona Q: Shadow of the Labyrinth.
After all this effort joining the universes together, it would only make sense for Persona 5 to continue this trend; furthermore, it should also tie in Persona 1 and Persona 2 since they are still almost without mention in the rest of the series at large. The incredible change in tone would also make it more appealing for interactions with the other Persona users from previous games to see how they react.