So a few years ago, Blizzard said they couldn't come up with ideas to trigger "hard modes" like Ulduar on every fight and that is why they didn't keep up that model.
However, reading the threads in here, people seems in a vast majority prefer that model instead of flipping a menu switch.
The last time Blizzard did it was in ToES, it wasn't really hard, but at least they did it, and they didn't even try this xpac despite a few opportunities to do it.
So for fun, let's see how they could have done it in Hellfire Citadel.
Hellfire Assault: Use the munitions only one one canon for the whole encounter, would trigger hard mode once that one is full.
Iron Reaver: Get it down to 50% before the first air phase.
Kormrok: Start at purple
Hellfire Council: Different Kill Order
Kilrogg: Soaking heartseeker using a hulking berserker
Gorefiend: Kill and heal to full all the adds inside the stomach before the first feast
Iskar: Kill adds in specific order
Socrethar: Do not use the construct
Valhari: Kill the fight with all 3 adds up
Zakuun: Let no pilar spawn
Xhul'harc: Keep the green add and purple add up together long enough so they merge with the boss to buff him.
Mannoroth: Do no kill any of the channelers (boss would be attackable)
Archimonde: Get him to x% before the first nether banish
You get the idea, sure somes are generic, but still better than just flipping a switch.
What would be your triggers?
Keep in mind it needs to be something that can't be triggered by a raid failure, like I thought of sending nobody in Kilrogg's visions or not interrupting something, but those are mistakes that could happened, it needs to be something you want to do on purpose.
Of course there are a few gear checks, those are the most obvious to do.
Could have been interesting to have some of those I must say.