During Gust's 20th Anniversary stream for the Atelier series (yes, it's been going on since the PlayStation 1 days), they announced the latest entry in the long-running franchise: Atelier Lydie & Soule: Alchemists of the Mysterious Painting. Yes, much like Atelier Escha & Logy, there will once again be two protagonists making an appearance in this game. All the developers had to share at the moment was the silhouettes of two girls.
We also know from the stream that Yuugen and NOCO will return as character designers. And like the other games in the franchise, this should mark as the bookend for the "Mysterious" subseries that has so far compromised of Atelier Sophie and Atelier Firis.
Unfortunately, that is all the details Gust shared during the event regarding Atelier Lydie & Soule, but we'll share more details once they are so kind to share them.