Instead of adding the difficulties of raid to M+, consider the opposite approach and make raids easier to form groups for and also eliminate loot lockouts (at least for LFR, Normal, and Heroic).
Making M+ require scheduling a week in advance, would simply kill M+ participation and popularity down from 19% to 9% ,where DF Raid is at per Blizzard’s own poll.
Killing the most popular “pillar of end-game content” will not make the community better overnight. It would probably kill subscriptions for those that enjoy M+ and don’t have the time commitment for Raid, therefore unlikely for Blizz to make that huge business mistake.
I doubt Blizz is going to make Raid more accessible as they seem to be of the mindset that “it’s not for pugs, it’s for dedicated guilds and we’re OK with it being less popular than M+ as Raid is the pinnacle of PvE challenge, so it’s not for everybody.” -_-