UPDATE 4/13: To our surprise it does look like this is going to be more than just one special episode. The spinoff series is slated for 10 episodes! You can stream now on Crunchyroll.
Ranking of Kings: Treasure Chest of Courage will be releasing tomorrow. To be clear, this is not season 2 but a special episode. This special episode does not have many details even though the release is very near. This is because the material is original & not from season 1 nor the manga. From the trailer, it seems to be light-hearted showing the slice-of-life part of the show.
Ranking of Kings synopsis: The story is about a little prince named Bojji who is deaf but above all naive, because despite many criticisms from his people about his accession to the throne, he does his best and dreams of becoming the greatest of kings.