Mogeko Castle Gaiden or General Hashasky's Great Adventure (ハスハスキー将軍の大冒険) will be one of Deep-Sea Prisoner's upcoming games and serve as a sequel to Mogeko Castle. A preview for the game is shown in the bonus room of Mogeko Castle at the end of the game.
Like the prequel, Mogeko Castle, this is rated by Deep-Sea Prisoner themselves as R-15G. Be warned, the game is a violent gag game and contains violence and other morbid things.
On the 11th of July 2018 YouTube user RozenKrieg uploaded an unlisted video showing the start of Cannibal Rhythm and Mogeko Castle Gaiden. On the 8th of August 2018, user Mogekov Hashasky also uploaded footage of Gaiden.
There are also numerous short gameplay videos on a seemingly-Russian channel with english translations (MinoLame something-or-other, https://www.youtube.com/channel/UCYhezo_iTe9VB8uJ-SWoRlw), the latest being 19th January 2019, which proves the game is still being worked on. (Seems to include a video that looks like the preview from RozenKrieg below, which is now a dead link)
Link to newest trailer (10 june 2020) : https://www.youtube.com/watch?v=B-eoSVHPRG0