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. 
I’m still here Birds!
Mythic+ end-of-run drops will cap at Mythic+10 throughout the week of March 1–7, and Season 3 Rated PvP gear upgrades are initially capped at Rival II (base item level 268) until Mythic Difficulty Sepulcher of the First Ones is available.
Completing the Great Vault participation requirements next week, March 1–7, will reward the new item level thresholds in the March 8 Great Vault after the weekly maintenance.
| Difficulty | End-of-Run | Missed Timer | Great Vault |
|---|---|---|---|
| Mythic+15 | ilvl 262 | ilvl 259 | ilvl 278 |
Class set pieces, excluding shoulder and chest, will appear in the Great Vault rewards starting on March 8 from all vault categories. Defeating raid bosses that drop tier will increase your chances of receiving that tier slot from the Raids row. The shoulder and chest pieces will appear in the Great Vault rewards from March 15 and beyond.
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