It’s been a subject of some discussion for the past few weeks and the original word came down when Blizzcon came out.
During the Q&A portion of the most recent BlizzCon’s Raid & Dungeon panel Scott Mercer, Lead Encounter Designer said:
“I know that UI mods are a huge part of raiding, and it’s something that, [...]

Having a Blizzard built in threat meter would be hands down one of the best additions to the default UI ever. Right now Omen and KTM are great (Omen being better by my personal experience) but they don’t really rely 100% on precise data and can be misleading or inaccurate.
If Blizzard were to add in [...]