I agree with Fenril. There's too many issues to think about and it will lead to too much "what's the 'right' version of X server?"

Do we get all the races in Vanilla that we have now? All the same classes? What about the roles those classes play? Does this mean pallies and shamans are both 3rd string tanks again? Druids will only be useful as resto? What about racials? Talent points and trees?

Do you go back to a world where a normal upgrade path included going from grey to green at level 50? Does this mean there's going to be 6 different versions of the same piece of armor to account for the Vanilla version, the BC version, the Wrath Version, the Cata Version, the mid-expansion stat redistribution....

Which version of quests? And Which version of the world?

What about things like functionality and addons? Do we get to go back to a vanilla where whispercasting still works and click+commas still work and addons can decide who to decurse?

Not only do those questions have to be answered, they also will have to be explained and all of it will need to be maintained. It's not really a matter of just plugging in the old saved game and loading from there like a console game.