There was a caveat to my old post that I didn't have time to add this morning:
Re-tuning
all the dungeons to have max-level counterparts which are all equivalent in terms of gear and tokens (justice/valour etc) would delay some of the burnout from running the same dungeons, but only delay it. And they must be equivalent; the worst example in my time in WoW was the constant rerunning of ZA/ZG five mans, because these were the most efficient in terms of rewarding token points for gear. The older WotLK dungeons still rewarded them but their age and difficulty, and thus loot and point rewards, were inferior so no one ran them, and because they'd already been run hundreds of time prior to the release ZA/ZG. Don't do that.
Maybe doing away with dungeons entirely and rebranding the pre-max game into a solo game, and "hiding" all group content until you're max level is the way to go. This pretty much happens already because anyone who's not leveling for the first time wants to get up to max level as fast as possible, because that's where all their friends are, and thus where all the fun is. As a result there really isn't many people left who want to
enjoy the dungeon experience; while the finder lets you run them without having to really organize everything, 3/5 of those people are there because they queued while leveling - they're not there to enjoy it.
But the issue there is the loot. You gotta give people a reason to put together competent raid groups (rather than just finding three dudes who feel like getting the Onyxia achievement, as my own experience went) and with the huge number of raids we're talking about (There's six vanilla raid dungeons I remember, if you don't count Naxx, and even more in BC and Northrend and Cata, and then we're gonna add all the Panda dungeons to that list in a few months) it might get a little unwieldy. How do you even set up a tier system for that? I doubt the ancient raiding guilds who've been doing this thing since vanilla are gonna be interested in having to plow their way through updated old dungeons (A lot of which weren't really all that interesting to begin with when you look at them without nostalgia goggles on) to qualify for better stuff.
As far as re-tuning the raids go, I'd re-tune them into five mans, and move all dungeon loot into token vendors, or duplicate items with various models. Kind of like the Headless Horseman's rings. (Some of the raids would also be broken for brevity, like they did with BRD/BRS.) You'll still have raid tiers for the current expansion, but all the old raids are broken into dungeons for your weekly heroic grind.I don't know how expensive the transmog system is, but aside from leveling it doesn't even really matter what items look like, because people can make them look like more or less whatever they want.