I would say an alternative path would be better. They should split leveling up like theyre doing professions where you can jump into current content or go back.
I'm sure they could "time walk" characters and gear to expansions and maybe unlock full level when they max the content. That would be fun as hell. Of coarse that would take a ton of programmer time and resources. And the lvl boost would be worthless...
Then again they did build an expansion once partially around redesigning old content.