As a preface:
This is written with intent toward someone from Blizzard reading it. Excuse the use of "you", it would be talking to someone who can actually impact the game.
When I first started playing this game in Vanilla, and I was dragged along to Molten Core to off-tank some adds for Majordomo. I immediately fell in love with the raiding aspect of this game. The assignment and acceptance of various roles, the requisite of attention to mechanics and placement were quite possibly the most amazing virtual thing I have ever experienced, even to this day. While I have never been a person to pay attention to lore, I mean I literally did not care which bosses were related to whom or why they were there in the first place, there was something truly immersive about killing Lady Vashj, Kael'thas, Archimonde, Illidan, Yogg'Saron, and the Lich King (probably because I had played through Warcraft 3 and it's expansion). I felt like I was actually taking part in something beyond the immediate raid scenario, as if I was contributing to a story. Nowadays, I'm not even sure who the bad guys are or why we're killing them. During heroic Deathwing, I could never get over the fact that I was really just fighting lava tentacles.
So here's my proposal, and it is neither an original nor ground breaking idea, but it might bring some challenge back to some of your players that have left the game and give the hardcore raiders of today something to do during all the off time between tiers.
It could use a combination of mechanics that are already implemented in various other titles. It will be a soft reset, like Diablo 2, for a certain expansion, say once every 2 months or so. Create a separate game client, much like the PTR, but back dated to various expansions. Then implement utilities that will streamline the raiding and keep it interesting for returning players.
1. Everyone has the option to create a pre-made 70 or 80 or 85 or whatever, with the ability to:
a) choose 2 professions that will be maxed out.
b) buy heroic 5-man gear from a vendor.
c) buy gems/enchants from different vendors.
d) remove reputation requirements for shoulder/helm enchants.
2. Once a group is assembled, do 10 man difficulty with 25 man loot. However, the difference here is that each person can choose one item from the loot table of that boss (tier tokens separate). The goal is to not make it impossible for people who are not experts, but still retain some difficulty. However you want to do legendaries, whether removed entirely or that you can collect enough pieces in one run is up to you. For every instance that requires rep to upgrade an item or buy something at exalted, make it so that the rep gained can get you to exalted within one run. The point is to remove farming and multiple runs. You can, however, reset a raid to go back for various achievements (see No. 4).
3. Create time frames much like you have with challenge mode dungeons today. Each raid has everything inaccessible until someone hits the "go" button. You could use time achieved on these realms to live realm rewards or just keep it open for fun. Maybe the Keepers of Time could set up challenges or something.
4. Once each tier has been completed, the raid can advance to the next tier to experience that. I'm not sure if it should be something like having to do various achievements, such as hardmodes, to advance, but something along those lines. Obviously Nightfall is a bit much of a challenge for most people.
This will do a number of things. It will allow people that have not seen previous content while it was current to experience it like some of us have. It will remove the grind from raiding that turns a lot of people off of it, and because it's old game clients, this won't effect live realms. It will give bored players something else to do. It will probably keep me subscribed and bring back old players. It will also very likely get rid of the rose colored glasses that seem to be so popular in trade chat. It will remove loot drama, expedite the process of gearing, and allow us to see how "hard" content actually used to be. This could be a chance for some of us to do progression on content that we enjoyed for a story that we cared about. I really don't want to resort to a private server to play a version of the game that I don't hate. Even with a reset every two months I would play the hell out of Burning Crusade and Wrath of the Lich King.
Let me know what you guys think.