With the flex scaling why not simply merge all dungeons and raids and have them scale from 5 to 40? Keep the Mythic at 20 and Challenge at 5.
And with the four leveling dungeons imagine your whole guild on the second or third day or the expansion gets together into a 14-man run into the leveling instance with 3 rare drops each boss instead of 1 with mechanics scaling acordingly. Or do an OpenRaid leveling event on WoD launch week with 40-man raids on those instances.