So what worked in Legion?
- Non-Linear Zone Choice
This expansion allowed the player to go through the four leveling zones in whatever order they wished. The NPCs there would scale with your character's level, and so would the rewards garnered. The reaction to this feature is overwhelmingly positive, despite a few deficits with its execution, so much so that when the new patch hit on Tuesday that spread this system to the rest of the game the arguments are about how to make it work- not about if it should be removed.
In a similar manner, each zone had a series of Chapters. (This is a hold-over from previous expansions.) You would pick one up from the first quest hub in the zone you began leveling in, and completing them all gave you an Achievement. It would also lead to a quest in one or more dungeons in that zone, with the objective to retrieve one of the MacGuffins driving the expansion's plot. You had the choice to do these Chapters as you liked, but many did them in a specific order to speed their way to completing as many content unlocks as possible.
No one complains about having the ability to complete content as they choose. Those seeking an optimized path can find a walkthrough with ease, while the rest of us do what we wish when and how. This is clearly a win for the devs, and it will be continued going forward; this too has been retro-actively applied to the older content with the new patch as a means of overhauling and improving the gameplay experience for new players.
- Mythic Plus
The two previous expansions had Challenge Mode dungeons, where you did a timed run at a standardized power level. Those who had the best times at the end of a season got exclusive cosmetic rewards it was all about prestige, not power. Participation was not where the devs and moneymen wanted, so they did an iterative improvement: they added an infinite escalating difficulty system, and then put in a series of modifiers known as "affixes" that change the rules. This is "Mythic Plus", and it rewarded power boosts in the form of higher-level gear for successful completion.
This was intended to be a parallel to raiding, aimed at those unable to make the Bowling Team commitment of raiding but wanting a challenge at endgame. It proved to be very popular, but not entirely as a substitute for raiding; many high-end raiders routinely did Mythic Plus to supplement their raiding. However, it did emerge as a thing to itself because it showed potential as an esport; the Mythic Plus Invitational tournament had Twitch viewership in the five digit figures, over ten times that for Arena PVP (the other esport thing Blizzard put on the official WOW Twitch channel).
Mythic Plus succeeded beyond expectations. There is an emerging subculture that focuses on Mythic Plus as a thing distinct from raiding, complete with external sites that track and rate players, and it's far easier to spectate than PVP is so there will be more formal tournaments in the years to come. This will be carried forward into the next expansion, hopefully with improvements where the rate of participation doesn't vary wildly from week to week due to what affixes are in play.
Then there's the clusterfucks. Next time.