Persona 5 The Royal will feature plenty of DLC and bonus content from the original game. The biggest fan addition is the ability to fight the Persona 3 Protagonist and Persona 4 Protagonist.
They can only be challenged in the Velvet Room so it is unclear on whether they have any importance in the story. Considering the Velvet Room has no effect in space and time, it most likely a fun addition. It looks to be a Optional Boss in the newly added challenge room.
The trailer showed the Persona 4 protagonist with his persona, Izanagi, and images from the Japanese Persona 5 Royal website showed the Persona 3 protagonist using Thanatos.
The female protagonist from P3P is not featured (as usual).
Persona 5 Royal releases in Japan on Oct. 31 and will come to the West in 2020 for PlayStation 4.