I believe a Challenge mode could work well for both old raids and dungeons in Vanilla and BC. Make those raids 5-mannable, and if you complete, say, all the vanilla ones in Gold time, you get some type of neat-o transmog gear modeled after the old tier set but with some kind of new flashiness. Same with the BC dungeons.
That, plus the time scores and such, could be a fun side thing to do. Especially for the nostalgia and memories with old friends and guildees running that content when it was current.
And, if you don't have Challenge mode triggered, the raid/dungeon stays as it is now.