When it was announced some time ago that the developers at Blizzard were thinking about introducing a rating system for Mythic-Plus dungeons in WoW, opinions were divided. Some players were happy that addons like Raider.io could become obsolete. Still other players were less than thrilled that such scores would now be thrust upon all players and that they might have an even greater (and possibly negative) impact as a result. Anyway, the time has come and the official Blizzard system has arrived on the PTR of Patch 9.1.
In doing so, it quickly becomes clear that Blizzard didn't go to great lengths to disguise its origins in the Raider.io addon.

  • You're assigned a value per dungeon, based on the level of the completed dungeon. These values together make up the Mythic+ Rating.

This rating will be displayed to every player who applies for your group through the tool. Conversely, if you are a potential applicant, you will also see the rating of the group leader of each group listed in the tool.
WoW: Official Mythic Plus Rating System Arrives on the PTR (1) Source: wowhead

Source: wowhead Source: wowhead

However, it seems that the system is not complete yet, as some features are missing. For example, Ion Hazzikostas had actually announced that there would be different ratings for Reinforced and Tyrannical. However, nothing of this can be seen on the PTR yet.

In addition, it remains unclear how exactly the value will be determined - how much the level and time will be weighted in each case. However, it is quite possible that Blizzard will use the Raider.io score as a guideline here as well.