Lots of class balance issues with mechanics problems (eg Balance Druid) impacting raid and Mythic+ desirability. What plan for 7.1?
Balance and number-wise is an ongoing process to make specs competitive. However each spec has strengths and weaknesses. For Balance Druids, things like Solar Beam and Typhoon are very useful for Mythic+ Dungeons (eg Solar Beaming imps in Court of Stars). Blizzard would prefer to double down and make those mechanics shine.
What about "bring the player not the class?" In many ways, Ion feels that class importance has been lessened too much. In Burning Crusade, the importance of class balance was an extreme jigsaw puzzle, and that was a problem. In the process of fixing that, Blizzard moved away from mandatory class setup.
Ideally, the playerbase should play all difference classes. The playerbase is best-served if they want some of everything. When it's just "we want a ranged dps" then raid leaders will look for the top-performing spec. But if specs have unique cool abilities, then raid leaders would be interested in multiple specs, not just one.
If all else is equal and you're picking a group among strangers, the community is better served when you pick specs that aren't already represented in filling out a group. There is definitely a lot of work to still get there! Perhaps by reintroducing abilities for utility or synergy.
With Mythic+ being a huge part of Legion, are there any plans to help specs that don't have strong consistent AoE so they can partake in higher Keystones?
The goal is to have areas of strength and weaknesses; eg a Demon Hunter can melt AoE while a Feral Druid can do single target DPS like no other. Consistent AoE is emphasized as a good attribute in strong farming clearing runs, or speedruns of lower-level Keystone runs.
However, at higher keystone levels with Fortified and Tyrannical, players are not clearing content quickly. Some affixes even make AOE a terrible idea. So other specs like Shadow Priests have a place to shine.
One challenge is community perception, ie it doesn't matter if you're good if people think your spec is bad and reject you from groups. This is a challenge for Blizzard; they are working towards improving visibility and perceptions as to how viable specs actually are. They recognize this is a huge factor in player experience.