I think the research has already been done on this.
The most popular Private Server models have already proven the best way to move forward with different expansions.
Just as Circe said, the classic, an in fact all the expansion versions can be distilled down into a single, and final progressed server of that version. Classic can be the final Naxx pre-TBC version of the original WoW.
New servers can be fired up each year at a particular time so that new players can start out fresh on that version of WoW. They can then be progressed to completion of that version and then merged as the population declines, into the final completed version of said expansion.
Additionally, fresh, untouched servers of a particular version could be launched and character transfers to these realms could be locked, āat least to a certain progression pointā to allow ONLY level 1 players wishing to start out and level on that expansion to cap.
As for moving from Classic WoW to TBC, I played a private server that had a quest line, completely developed, that once completed the level 60 classic character walked through the dark portal in the Blasted Lands and at that point, there character was forever transferred forward (no going back to classic with that character) to the TBC version for that server/realm.
You could start level 1 characters on the TBC server, but the majority of players came from the previous progressed server.
It does not take a great deal of effort to create multiple realms and list or name them in a manor that players can choose how they wish to progress.
I think the main concern this addresses is letting people that want to stay in classic and never move forward, the option to simply not do the quest line that starts the move to the new version of WoW.
It gives everyone time to progress at their own pace and enjoy this great game on their own schedule!
Again, the key here is to have battle groups for PvP broken up into progression / patch stages.
Every server / realm created, is progressed into that final patch for that version of WoW and then the population is merged with the original final progressed servers/realms. This allows the new players to come to always be able to experience the original. Starting from the first phase on the anniversary of the first original launch day!