I see people saying how bad they want normals/heroics to become apart of the Flexible system. This would be a horrible...horrible idea.
First, let's look at why it is a terrible idea for only normals to become flexible. Let's say a raid of 14 people, spend weeks (even months) progressing through normal mode. They spend so much time and effort with each other to clear the instance on normal. These days, a group of normal raiders progressing through normals plan to go to heroic progression with each as well. If the system were flexible then a lot of poor souls would be kicked out of the main group once the guild got to heroic progression then next thing you know people would be begging heroics to become flexible as well.
Now let's take a look at if heroic raiding were flexible as well. The idea of heroic raid progression is to make the fight as simple and as easy as possible to get the kill. You know that "sweet" number in flex which is like 14 or so people but it doesn't really matter because of how easy flex difficulty is atm? Well it would matter in heroic raiding. Heroic progression guilds would only push for that "sweet" spot to ensure the fight stays as simple as possible to better their chances at getting the kill. The "sweet" spot could change depending on the mechanics of the boss and how they scale with flex etc etc. So in the long run, flex would not work with heroic because people progressing will always go for that one specific number that makes the boss fight easier to manage.
TLDR - Leave normal and heroic raiding alone.