They are casting right now for BB 18.5.
All of the all access seasons are gonna be shortened in length and are going to be used to both test future comps and twist ideas.
Each season will air for 5-6 weeks and feature 9 house guests. Every houseguest will make jury and rumor is they are looking to cast new types of people (age especially) because they want to see how older people would play the game of there were 4+ 40 year olds in the house.
Over all BB 18.5, 19.5, etc are gonna be experiment seasons for future full BB seasons