The game won't be an existing IP, but they'll take their experience on Final Fantasy XV and apply it to this "high quality" title.
This week Episode Ardyn wrapped up the DLC content for Final Fantasy XV, and in a recent interview with Famitsu Luminous Productions spoke about the episode in great depth. What's more is that they also talked about the future for the team, and what they're working on now.
The team that helped build Final Fantasy XV is working on a new AAA title that's not an existing IP, which is the whole reason the studio was born, and while the team can't talk about details, they say that the emphasis is once again on "high quality". We also get confirmation that the next project "is already in motion".
Back in November last year we found out that Luminous is working on a "new AAA title for PS5", and that they are focused on "large-scale, high-quality AAA game titles," so while this latest statement isn't exactly breaking news, its a reassurance that these projects are in the pipeline.