I posted this on the WoW forums. I think speed running and 5 man mythics should be separate parts of the game. They need different skill sets, speed running is about minimaxing dps and often cheesing mechanics, normal mythics (at least when people don't overgear them) require thinking. Not to say timed runs can't be hard.
Mythic0 is already easy for most people. My idea is to make mythic0 genuinely hard and award loot 5ilvs below heroic raid gear (because forming a raid is harder than forming a group). Make them as hard as heroic raids.
Have heroic dungeons have a timed options with all the affixes we know now. What would they drop? Maybe items that helped complete dungeons faster, cosmetic/mount rewards and at very high keys maybe some BiS gear that helps in raids.
Tl;dr just separate timed and untimed content, I hate that 5 man content is funnelled into speed running which in my opinion fosters elitism and ilv obsessing.