My fiancee and I have both agreed that if they don't bring back strict 10-man raiding, that we're not buying Legion. 20-man raiding, while looking good on paper, was the ultimate nail in the coffin for many guilds this expansion, even ones that already had a large, active roster. I despise larger raid groups because it's easier for people to get carried, and you lose some of the tight-knit, social aspects that you can only find in smaller groups.
Also, as the game continues to bleed subs, it will be even harder for 20-man Mythic guilds to find decent recruits.