With the changes announced at Blizzcon yesterday the raiding environment could change even more. Mythical mode gear could become something like the arena raiting gated gear for raiders. It should have the same ilvl as the heroic mode drops, but with a different color.
Mythical mode will be the biggest challenge for players. Why should they destroy it with better gear drops and nerf it for you every week. They should even add ilvl scaling to it after the next raid patch is released to keep the challenge alive.
The rewards for mythical mode would be:
Different colored gear no casual could ever get his hands on.
160+ Achievement points for all the bosses in the first tier.
Cutting Edge FoS (with a special title like hellscreams downfall)
1. Only 3 ilvl per tier of raiding. Smaller difference between casuals and mythical raiders.
2. Easier access to mythical raiding, because there is no ilvl barrier.
3. Makes hardcore raiding a skill check instead of a gear check.