Square Enix has confirmed Nier Replicant's extra content via a new trailer.
In the video, below, we see an extra dungeons, extra costumes, the guest cast from Nier: Automata, and a look at an extra episode called Mermaid.
15 Nightmares - aka The World of Recycled Vessel - first released as DLC for the original 2010 action adventure. This is now included in the main, remastered game.
Speaking of costumes and weapons, Nier Replicant has Kabuki and Samurai costumes, and costumes and weapons from Nier: Automata. There's a guest cast and soundtrack from Nier: Automata, too.
The main draw, perhaps, is the Mermaid extra episode, which is set on a wrecked ship and features a giant shade.
Nier Replicant, or, to give it its full title, NieR Replicant ver.1.22474487139..., is due out on 23rd April on PC, PlayStation 4 and Xbox One.