Raids

Raids have been the backbone of WoW since classic - its USP, its most exclusive and most intricately designed content, the challenge that separated the wheat from the chaff, the center of all balancing efforts. There is little I can say, except one thing: 

With WoD, one very fundamental mistake was made, which was basically a return to BC-days: the most demanding content was restricted to larger groups. The change of mythic raids to 20 men groups singlehandedly broke the back of plenty of ambitious raiding guilds and sucked the fun out of the game for countless smaller mythic raid groups. Groups which up until that point were able to progress in the endgame content with a roster of a dozen people were now forced to at least double it in size, which meant a lot of searching for qualified raiders who were willing to submit their spare time to a raiding schedule, had the proper class, gear and got along with the other members. Plenty groups didn't make the cut because people left the game or were poached by other raids, scheduled runs weren't possible because 2-3 people were missing but couldn't be replaced at the last minute, mediocre players had to be accepted because a full group was still better than none at all etc.

While it's understandable that a complete adaption of the flex raid system isn't really feasible for mythic raiding, there are at least some options to make that mode more lenient:

•  instead of 20 people only, small group mythic raiding gets reintroduced, i.e. 10/20 or 10/25 are possible raid sizes. This opens up this content for smaller raid groups again - and it's hardly too much to ask considering that high-end raiding holds considerable appeal for a certain segment of the playerbase.
• semi-flexible raiding: mythic raid sizes are a bit loosened up - they can vary from -10% to +10% (i.e. 18-22), with identical boss mechanics, but daMage and hp being adjusted. That way it isn't the end of the world if a few people are missing.
•  or both.