Persona 5: The Royal will include DLC after release which will bring back the protagonists of Persona 3 and Persona 4 to take on the Phantom Thieves of Hearts in boss battles.
The unique identity of the Persona series was cemented with the release of Persona 3, as the first two games were more similar to the Shin Megami Tensei series than the later games. It's for this reason that the later Persona titles have crossed over which other on different occasions, such as in Persona 4 Arena Ultimax and the Persona Q games. The SEES members, the Investigation Team, and the Phantom Thieves of Hearts have already met in previous titles.
There will be a new crossover between the Persona series in Persona 5 Royal, as the latest Morgana Report on Atlus' YouTube channel has revealed that Makoto Yuki from Persona 3 and Yu Narukami from Persona 4 will appear as DLC bosses in the game.
Persona 5 was severely lacking in post-game content in comparison to the other games in the series, as it only had a single superboss battle. The fact that the Persona 3 and Persona 4 protagonists are fighting alone suggests that they will be acting as incredibly challenging superbosses.
The new trailer also revealed the "My Palace" feature, which is an area that the player can decorate themselves. The player can earn new furnishings by completing challenges within the game. There are also new costumes coming to Persona 5 Royal as DLC, including the cartoonish appearances from Persona Q2: New Cinema Labyrinth.
It's a shame that Persona 5 Royal is using DLC, as the game is meant to be the definitive version of Persona 5. The extent of the new material in Persona 5 Royal has yet to be measured and its unclear how much post-story content will be included in the game. If Persona 5 Royal is lacking in superbosses or post-game dungeons, then fans will be annoyed if they have to shell out cash for the battles against the Persona 3 and Persona 4 protagonists in order to face some challenging bosses.
Persona 5 Royal will be available in Japan for the PlayStation 4 beginning October 31, 2019, and in the West on an unspecified date in spring 2020.
Source: Atlus/ YouTube