There was a Dev Q&A on Reddit last night. Below is a question about Ulduar type hard modes. I've love they way they came up with Mimiron's hard mode trigger. "Why don't we put a big red button on the wall." Hey, that's a good idea.
"Q. I always felt the Ulduar raid was the best design for including hard modes that were mostly built-in to the encounter versus the normal mode-hard mode toggle that we have now. Do we have a chance to see a return of the event-based hard mode design instead of a toggle"?
A. [–]WatcherDev 91 points 12 hours ago
The Protectors of the Endless encounter in the Terrace of Endless Spring raid in 5.0 works exactly like the Iron Council hardmode from Ulduar; if you defeat Protector Kaolan last on that encounter, you get "Elite" loot that is even more powerful than the normal rewards from the encounter.
It's the sort of thing we'd like to continue to experiment with and introduce where it makes sense. For some of the Ulduar encounters it was intuitive (kill Freya while her protectors are still alive), but in other cases it was obscure (how many people would have discovered the Vezax hardmode without an achievement describing what to do?) or an outright trap ("Why is XT hitting so hard?"). We literally spent as much time arguing over how to trigger the Mimiron hardmode as we did designing the fight itself, before someone half-jokingly suggested, "what if we just put a big red button on the wall?"
So the toggle is here to stay, but we're definitely keeping our eyes open for places where it makes sense to apply the Ulduar model.