Who else would like this?
Each week there would be a random old raid that you could run together with your guild, and you would all be scaled down to match that raid (or vice versa).
Completing that specific weeks raid would net you a stash of that weeks valor. I'm not sure if a few random LFR loots thrown in would give people more incentive or if that would be unneeded.
IT would give people a lot more excuse to visit old raids as they were intended to be seen, there would be a whole heap of variety too if we included all raids from the history of WoW. (Plus dungeons would remain the usual current difficulty at other times)
the technology is pretty much there already, with all the scaling, so I can't imagine it would be THAT hard to implement.