Hello Krystallíne!
When Season 3 begins next week, Mythic+ Keystones will be lowered by an additional 3 levels to account for the overall increase in dungeon difficulty. If you’d like to open your Great Vault next week and earn a +15 Keystone, you’ll need to time a +18 or complete a +19 untimed before next weekly regional maintenance.
Hope this helps clear things up. 