Neptunia has been a series that has been on more or less a hiatus on mainline games ever since they finished making their game Megadimension Neptunia VII. Today Idea Factory and Compile Heart has released a new website title. With that new website, we also have been given a teaser trailer.
The majority of the video is just Nepgear, but it ends with a giant date of 11/25/2021.
From what we can see is they hint at a new game that focuses on the sisters. There are some that theorize that it could be Reverse 2. Neptunia Reverse was released earlier this year and was a remake of their remaster titled Hyperdimension Neptunia Re:Birth. Reverse 2 could be a remake of the other title Hyperdimension Neptunia Re:Birth 2, which also focuses on the sister characters to the main characters.
The main characters in Neptunia generally are Neptune, Noire, Vert, and Blanc. Their sisters are Nepgear, Uni, Ram, and Rom. This is a series that is basically a satire of the console wars with characters that represent the main consoles of the time period they were in. Neptune is Sega, Noire is PlayStation, Vert is Xbox, and Blanc is Nintendo. It is more hijinks and comedy than trying to tell a big-time story but it does it well enough to have a cult-like following.
Another possibility is that it could be a mainline title that follows Megadimension Neptunia VII as it says in Japanese (A new main entry) in the video. We won’t know until they reveal more information on November 25th, 2021.