This is the best announcement of all time. The two best dungeon crawlers ever made in Japan finally meet!
I'm not sure if this game is totally developed by Atlus btw, looking at the trailer, it Spike Chunsoft is pretty heavily involved. The producer is the EO series director Shigeo Komori, while there are two directors. Yukari Yokoro is the Atlus director, she's an art designer on the EO series, while Seiichiro Nagahata is the Spike Chunsoft director, he directs and co-directs all the Mysterious Dungeon games iirc, including the Pokemon ones.
The character designs are also a split, with the regular EO character designer doing the base character designs, and the Shiren character designer doing the finalized designs. The music staff and monster/art design are all EO regulars.