We know that Persona 5: The Royal is set to be revealed later this month on April 24, but it appears there's another Persona-related announcement on the cards, the very next day. 'P5S' is a project that's been teased, with an announcement set for the day after the Persona 5: The Royal reveal.
Calling Card - April 25, 2019.
We will show you a new Phantom Thieves that will turn the world’s cognition on its head. We are energized by voices that seek satisfaction. Once again, we will take your heart at the Ryougoku Kokugikan. —From the Phantom Thieves.
It appears the P5S website was set live on April 2, so don't worry about it being an elaborate April Fool's Day joke. Note that Persona Super Live, a live concert that takes place in April every year in Japan, is set to run from April 24 to April 25. The announcements could be made as part of this event.
This ties in with a previous domain registration by Atlus from December 2018. P5S, as well as 'P5M' and 'P5B' were registered as domains by Atlus at the end of last year. While there's no word on what the latter two domains could refer to, it appears as though Atlus is getting ready to reveal P5S, at least.
In other Persona 5-related news, Persona 5: The Royal was finally confirmed by Atlus last month. All we know about the project to date is that it has a new female character, who could possibly act as a new playable character similar to how Persona 3's female protagonist was introduced. We'll have to wait until Persona Super Live, at the end of this month, to find out more.