Hmm I like this idea But blizzard wouldn't do it, Like blizzard have said many times, they want the game to keep moving forward other than going back to old content
We are aware and do acknowledge that there are people would like us to focus some more on renewing old content, but we do indeed want the game to progress and evolve with new content. :-)
It has previously been said that adding new content is something we feel is generally more exciting than renewing old content, and this is still true today. Renewing existing content can be great in some situations, especially if it is content that is very popular or if it is content that hasn't been experienced by a large portion of the player-base, but in generally we feel it is better to spend our resources on creating new content that is fresh for everyone.
Existing content has been renewed before though, and it is most likely going to happen again in the future in some form. Even though there are no immediate plans for it, perhaps we will one day get to see the Wrathgate and the fight for the Undercity converted into scenarios :-)