So basically, I was thinking, with the way they're going into 20man raids as the only viable raid format for Mythic, do you think a new 10man raid would be a viable idea, however not a loot dropping raid, a challenge raid. Note: It's a side raid, similar to what Karazhan was.
Imagine say, it has three separate difficulty settings: Bronze. Silver. Gold.
There are say, 8 bosses, and each boss drops <ONE> token (Prot/Vanq/Conq), that can be turned in for a piece of a new transmog set, different colourations for the three difficulties.
As a Challenge Raid, your gear is scaled to iLvl X.
On Bronze, bosses are tuned to be challenging for players that find Flex 3/4 to be a slight challenge, ie. harder than Flex 3/4.
On Silver, it's tuned to be challenging for players that find Normal last 6 to be a slight challenge, ie. harder than Normal, easier than the later heroics.
On Gold, it's tuned to be harder than or equal to Heroic Siegecrafter/Rag/Garrosh/LK on each boss.
Because it's a challenge raid, they could easily get the scaling to be right, and it would have to be run multiple times for people to complete their sets, resulting in a large amount of content, of course there'd have to be a large differentiation between the gear sets in colouring so they're distinguishably, and perhaps instead of a boss that drops a bracer token, instead a boss that drops a weapon or off-hand/shield token that could also be turned in for your choice.
As a Challenge Raid also, that's not bound by gear, it could be cross realm to line up with Blizzard's new oQueue/OpenRaid feature that they're implementing INGAME, and the balancing could be tuned around a standard, 2 tank, 6 DPS, 2 heal makeup, obviously this would be loose enough to be 3 healed on Bronze and perhaps Silver.
Loot would be traditional 1 token MasterLooted format, and of course you can only run it once per week per difficulty, so you can't just carry people through for their tokens.
Another thing to note would be that, in the example of MoP iLvls, I found with Challenge Modes and Proving Grounds that scaling to the basic Heroic 463 level really limits your choices as to builds for different classes and limits your capabilities. Thus the iLvl scaling of it could be say that, it scales all your gear UP or DOWN to a level, say 500. If your gear is above 500, it gets scaled to 500, if your gear is 463, it gets scaled to 500, if your gear is below 463, eg. 450, it gets scaled to 487, thus you still need a minimum of 463 iLvl gear in every slot to get the maximum out of it, but it's not restrictive in that you have to raid to get a high enough iLvl for it.
Also note, it doesn't necessarily have to be 10man, it could be 15, 20 or even 25, but as a side raid with a Challenge aspect to it, I think 10man would suit right with 20mans, it allows Mythic guilds to split themselves into two to do this content on off days say, it allows LFR/Flex/Normal players to throw together a small group, but still a raid group to take part in the content.
And one more thing, perhaps transmog isn't a 100% idea to encapsulate all players, of course another bonus could also be by completion of this raid, it awards you 500 VP.
NOTE: When I say Challenge, it doesn't necessarily have to be timed, because it's scaled so tightly around a static iLvl, the bosses especially on the Gold setting would be hard enough that, even a group that farms it on a regular basis would wipe quite a few times on each boss even during re-clears. On Silver/Bronze, it would be easier, but still challenging of course.
That being said, you could tie in extra bonuses for completing all 8 bosses faster, ie. a bonus token piece, extra valor, or something else etc.
I feel that something like this has a place, though maybe not in the exact form that I've outlined here.
My initial imagination of this was that something like, the plane of Ice that Ahune comes from, perhaps we even fight Ahune, thus all the sets could be designed in a similar theme but take into account aspects of each class, ie. lots of ice/cold.
Or perhaps other areas that are around in WoW.