Originally Posted by
Polygnome
We already have that system somewhat.
From LFR to Normal bosses gain abilities.
From Normal to HC bosses gain abilities.
From HC to Mythic bosses gain abilities.
The current M+ Affixes are simply unusable for raids.
In single-target encounters, Necrotic would just be your garden-variety tank-swap debuff. But in fights like Skopyron or Aluriel, or P3 Augur it would wreack complete havoc on the tanks. Simply not possible. Its works in M+ because you rarely have adds in boss encounters and because you get outfight between trash packs so that it resets. This concept does not translate well to 8+ minute raid encounters.
Other affixes have similar problems. Volcanic is just your garden-variety "run out of this shit" void zone, but we already got those on raid encounters. There is no need or reason to add them with an affix (it would make certain fights ridiculous). And if you remove them from lower diffculties, the bosses really become just loot pinatas.
Raging would be a catastrophe. In M+ you can stun trash packs that start to rage and can burn down bosses with HT and cycling of def CDs during the last 30%. This obviuosly does not work for longer raid encounters where sub-30% often still is 2+ minutes.
And the list goes on and on. Due to the non-timed nature of raids, Fortified would be absolutely irrelevant on Trash, and make add-heavy encounters just impossible or tremedously easy when the affix is not present.
Raid bosses are tuned around their abilities, which kind of already are the affixes. They just don't change over time, the change based on the boss, and the boss is tuned around that. In order to allow for raid affixes, every boss would need to be just a Patchwerk boss, and then get the abilities via the affixes (kinda like M+ bosses). Which is dull and boring. The only reason it keeps M+ entertaining is because the fights are short and its timed.