Maybe we need a Cataclysm

[Shurrikhan]

I'm not sure what you mean by this. You don't encounter N'zoth until you get inside the raid. Meanwhile, you can experience the corruption effect, including gears that can corrupt, before you even do the 8.3 starting quest that gets you the cloak. In fact, in the beginning, people in LFR can even go to N'zoth without getting the cloak before Blizzard fixed it eventually.

I'm referring to the entity in the plot, not just the final boss fight (or trivialized LFR version thereof). My alts have no trouble leveling through Uldum just because that zone is later reused for endgame content.

Nzoth had already spoken to us (literally getting in our heads) well before the 8.3 starting quest. Heck, his name's been tossed around a fair bit since the previous expansion and was a prominent lore figure behind the both the 8.1 and 8.2 raid series. He was even featured in a cinematic before BfA was released.

He's not exactly "an unknown" prior to his fight; he's the central antagonist of the whole expansion, just acting through various other agents until that final boss encounter.

For example, you can change the FATE system in this game and tie it to an event that happens at MSQ level 80, but if you do that, it does not make any sense for that change to be experienced by people who have not done that quest.

It absolutely can. You can build up cooler uses of FATE's central mechanics, allowing for -- for once -- a truly interesting or difficult FATE, and then trickle those interesting mechanics down across all prior FATEs. You don't need 'Latest Ascian Boi X' to feature prominently in each of those FATEs for them to make use of the tech.

Similarly, you can develop, say, more interesting gearing or upgrade/augmentation systems as a central feature for a new raid tier, but still make use of parts of those concepts (with the lore in no way conflicted) across the game as a whole.

Anyways, after rereading the OP, I hope that the roulette for tomestones isn't going to change. The point is to give max level players a reason to do old duties and help others queue, so I'm not sure how a calamity is supposed to change that while still helping out the queue.

Same, honestly. I mean, I do want to see an actual Expert Roulette at some point, and content upscaling, and a ton of other features to make more efficient use of development time as considered over more than just the latest patch, but the purpose of roulettes is a good one.