This was originally posted in a thread about Mythic being exclusive to 20-man groups, but it probably deserves its own thread because it's not about Legion specifically.
Endgame needs a whole new approach.
When push comes to shove, there's no change Blizzard can make that won't piss people off. We got here because of never-ending tinkers with a system that was probably broken to begin with, and we're now approaching the stage where tweaks won't be as effective as simply tearing the bandage off altogether and treating the wound underneath properly. The opening up of raiding in Wrath of the Lich King was arguably the best decision the game ever made, but it was clear that "real" raiding remained the practice of people in 25-man guilds, and these guilds were much better for the community as a whole.
In Cataclysm, the community was splintered because of a reward-sharing approach that was disproportionately skewed in one direction; toward 10-man guilds. Mists of Pandaria continued this, cementing the preeminence of 10-man guilds, only for Warlords of Draenor to enforce at 20-man setting that gutted the 10-man scene.
To be honest, content provision for groups needs a ground-up series of modernizations, and an encompassing approach that includes everyone.
For example, skirmishes could be designed for three to six players; dungeons could be designed for eight to sixteen players; raids could be designed for sixteen to thirty players. Rather than a difficulty slider, the instance itself should have a difficulty curve that increases as it moves on, culminating in what is now heroic difficulty. Individual bosses in either skirmishes, dungeons or raids could be tuned beyond that, assuming the flexible size of group isn't a roadblock. The never-ending difficulty slider needs to go, and players should be given fresh content at a significantly better pace than it's delivered at now, with major patches alternating between what's provided.
Remove the traditional idea of "tiers", where you gear up to BiS on every one. Have expansions last two years, with a patch every four months.
x.0: Launch content.
x.1: A skirmish and two raids.
x.2: Two dungeons and a raid.
x.3: Two skirmishes and a raid.
x.4: Two skirmishes and a dungeon.
x.5: Two dungeons and a raid.
x+1.0: New expansion.
Given that people will suggest that such a content rate is impossible, ignoring that other studios can almost do it on a fraction of Blizzard's resources, you could still have a two-year expansion cycle with six-montly patches.
x.0: Launch content.
x.1: A skirmish and a raid.
x.2: A skirmish and a dungeon.
x.3: A dungeon and a raid.
x+1.0: New expansion.
Under this model, the question isn't about difficulty - it's about whether or not you want to play all of the content. If you're in a really small guild, you get challenging group content in major patches; if you're in a medium-sized guild, you get challenging group content in the form of skirmishes and dungeons; if you're in a big guild, you get to play everything and the incentive for guilds to expand is obvious.
Unfortunately, nothing like this will even be considered. There are lots of reasons but, mainly, it all falls down to two things:
1) The game is old, and this is a fundamental change in how endgame is delivered by a group of developers resistant to change.
2) The community cannot accept the fact that not everybody can have everything, or that what's good for the game might not be good for them.
Sadly, even if a new game was to go this route, we'd still see a community argue with itself over what it does, and does not, have access to.
As gamers, frankly, we deserve much better than we get.