Blizzard has confirmed what keystone level players will need to complete this week in order to get a +15 keystone for the start of Season 3! This means that you should complete one of the following this week:
A +18 Timed Keystone A +19 Untimed Keystone 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. Read more on wowhead.com