Yeah... I don't see it happening, to be honest.
They would have to create new models and animations that matches the look and feel of classic, it can't be pretty like it is in retail.
In addition to that, the pacing and overall design for raids are drastically different as well. Just take a look at Naxxramas and how much it changed from Vanilla to WotLK. People would pick on the smallest little details on a brand new raid and say it doesn't belong in classic.