Waiting for this too, and some blue said, some time ago, that they could expand on this lateri see this kinda thing on the horizon tbh
with LFR and LFG and a little bit of the challenge mode gear normalisation mechanic, you could essentially scale any player back to level 60, 70, 80 and 85 to tackle old heroics and raids as they were intended (depending on what pre-set gear levels the developers decide upon should such a thing be implemented)
it'd certainly make transmog runs a bit more interesting as well as keep all the old dungeons and raids as "current" content, but probably not wise to use them as progression content, just something like scenarios and a way to get points to buy entry level raid gear.
for example, i'd love to be able to go back and do molten core 40 man, onyxia 40, naxx 40 and AQ 40, i missed all the 40 man raids as i started in TBC, being able to use the raid finder to go back and give them a crack for some transmog gear and a bit of gold would be awesome. same for sunwell, even if it was a slightly nerfed version (to cater for the fact it was done through lfr), i'd love to be able to go back and raid it with 25 people.
Challenge Modes are the first try