Three Raids, One Tier
The first raid tier was split up into multiple instances because the fights and their story takes place across a couple different locations: Dreamrift is part of the story, but it's not part of the campaign, while the final fight of the Voidspire is a part of the main campaign which will take players into the March on Quel'Danas.
Of course, players just playing for the campaign will still be able to see and do everything on Story Mode too.
Balancing three raids isn't a whole lot different from how they'd normally balance just one, but the split is more approachable for casual groups trying to decide where to spend their time. The developers think it will make pugging more interesting, especially with the way Mythic lockouts work. It's nice to be able to change up the format.
Cutting back on addons has resulted on a greater focus on visual clarity for things players would have depended on addons to keep track of. They also just wanted more of the gameplay to happen in the world and between players. Chat and pings are more interesting forms of communication than having an addon tell players where to be at all times.
There's going to be a pursuit of optimization with Mythic raids that feels like the use of certain classes is required, even if its technically doable without. Normal and Heroic are tested to ensure they can be done without them.
Dungeon Design
The developers have seen feedback that folks enjoy being able to work towards unlocking each level of resilient keys, so no changes planned there. The focus in Season 1 is on lower difficulties, with the new guidance affix helping on-ramp newer players and especially tanks.
Developers have discovered that 30-minutes is a good baseline for Mythic+, since 40+ minutes is a long time to ask players to stay switched on for. They also want to ensure that dungeons have similar runtimes within the same season, since outliers tend to get skipped if they're too much longer or binged if they're noticeably shorter.
There aren't any plans to specifically remove RP from dungeons, though they have been known to trim it down for keystones. More important is making sure that the amount of RP time in a keystone dungeon is going to be the same each time, so that there's no variance.
Three dungeons are part of the level up campaign, as opposed to one in The War Within. They want it to feel like part of the expansion story, though past expansions often treated them separately due to being group content. The advent of follower mode has given pretty much everyone the ability to go in and complete a dungeon, so they can be used as more narrative spaces.
Part of the goal of tuning the Mythic+ rewards system is making sure that the highest tiers are still available and fun, without feeling mandatory because of their rewards. They experimented with changing this on the Beta, but felt that there hasn't been enough change in the expectations of Mythic+ to warrant increasing that "mandatory" point higher.
There are all sorts of metrics for determining if a season is successful, which largely boil down to measuring player sentiment: whether they're having a good time. Dungeon durations, success rates, and determining win rates for the new PvP training grounds feature. While well-matched PvP is meant to have around a 50% win-rate, training mode can afford to be higher, since it's solely against AI - it shouldn't be 99.9% because they don't want players to afk and still win, but they'll use metrics like that to assess the difficulty and success of the mode.
Story of an Ongoing Saga
One of the great things about Midnight is that it's part of a trilogy. It's chapter 2 of the World Soul Saga, created by the same team that's been building story over the last several years, and will carry forward into The Last Titan. There are some intentional story beats being setup that won't pay off later, as dominoes that were setup long ago finally fall.
The name of the expansion is Midnight, so there is some expectation that its a darker, middle chapter. They're already building off of some fairly dark story in The War Within, which culminated in Xal'atath summoning the Void Storm to kick off her assault, so it is starting off somewhat dark. This is World of Warcraft though, so they try to balance the drama with a bit of levity; features like the Arcantina or player housing neighborhoods may generate some of their own drama, but there aren't going to be subject to any void incursions.
No spoilers, but forging will play a role in the campaign of Midnight. Won't say whether there will be a new Quel'Serrar or Quel'Delar type weapon, but we will end up working with Blood Elven forges to piece togehter relics of the past in a meaningful story building way.
Players will earn a piece of decor by killing all the rares in Zul'Aman. You can already see it in a couple places in the zone, but it's huge: you could put it in your front yard and place your whole house on top of it.
