I know this is the equivalent of asking for solo queue, and I know the usual retort from Blizzard that only at 20 you can cover the full spectrum of mechanics/buffs/debuffs BUT with the game going the way it is PvE is turning in recruiting and gearing up/skilling up game more than actual content progression.
This is especially felt at a point of Mythic raid entry level guilds who have a hard time attracting players of the right skill and gear level as anyone geared or skilled will just jump to more progressed guild.
Guild I am in specifically is entering a third or so cycle where we are within a reach of entering Mythic raid only for few core members deciding the wait is too long and moving onto more progressed guild only for us to again having to make a decision whether we fold, gear up/skill up subpar applicants or accept we can only be AOTC heroic guild not because the core does not have the skill but because we simply can’t get the numbers.
What stops Blizzard from introducing a bracket between heroic flex and mythic, which would be mythic flex (or even the old Wotlk 10man raid)? The world race can still happen in the confines of 20 man raid and everyone else can do their own thing. Even the old argument of loot no longer holds up because in most cases you can get equivalent or better loot from outside of Mythic raid, the rigidity of Mythic being set to 20 now simply gates people from content for no particular reason. Blizzard continuously boasts new systems implement yet to date they cannot figure out how to flex the mythic difficulty?