As one raid tier ends and we look forward to the next one, I've been thinking about the end boss of Castle Nathria, Sire Denathrius.
I think Sire Denathrius was an excellent end boss for a raid. He was a difficult fight, which took us many attempts to learn and master. But at the same time, Denathrius was a predictable fight. Abilities happened on schedule, and there were definite marks the raid aimed for and tried to hit. Once we killed him once, and knew we could kill him, we repeated the kill every week. If we failed to kill him on an attempt, it was always because we made a mistake somewhere in the fight.
At the same time, though, Sire Denathrius never really became easier to kill. Killing him every week felt as difficult as killing him the first time. We would always switch from alts to mains and make sure we had the full complement of healers. Heck, one of our tanks switched classes after the first kill to take less damage and make repeating the kill easier! We couldn't really carry anyone on the kill. We only got AOTC for our two casual raiders (who don't come for progression night) in the second-last week, even though they had been faithfully attending every week.
I think the primary cause of this was the lack of titanforging. As a raid group, we killed Sire Denathrius near the peak of our gear level. Obviously we got some better gear from Denathrius and filled out holes, but we didn't improve enough to really put him on farm. In the two previous expansions, titanforged pieces or corrupted items would continue to drop. The raid's power level continued to increasing, making farming the end boss easier and easier.
I think that aspect of titanforging was useful. It made farming less stressful and more casual. People could bring alts, or we could carry friends, or take non-optimal compositions. I'm pretty sure we were two-healing an 18-person heroic N'Zoth by the end of the tier.
Maybe taking advantage of the Vault would work. For example, if you killed Sire Denathrius, your vault would guarantee one piece of gear from the next tier. So if you killed Heroic Sire Denathrius, you would get a Mythic item in the Vault next week. A Normal kill guarantees a Heroic piece. I think that would be enough to preserve the challenge of the first kill, but still allow the raid to improve enough in three to four weeks to make farming the end boss more relaxing and allow for more slack.