I'm not sure if getting to the end of Vanilla then leaving it there is the end game here. But if we do go to TBC and possibly further expansions I have some suggestions, assuming they weren't just common sense, in which case disregard this and keep up the good work.
Firstly instead of making an expansion, make it as it's own release server, that way the people who do want to stay with just vanilla can do just that. Or return to it if they get bored of TBC or further expansions.
And seconds, since they will all be Elysium operated, while some people can choose to level 1-70, make it possible to import copies of characters from one server to the other. As in a level 60 on Elysium will still have that character on Elysium, but an exact copy will be pasted over to the TBC server ready to immediately get on with Outland content.
Once again, not sure if this will ever even happen, but if it did it would be nice to have these things idea's implemented, otherwise I imagine a lot of players may be put off the idea and not play TBC or quit Elysium altogether