This sounds like a neat idea for sure but I could see a lot of problems with it. This would require a lot of servers dedicated to this, which I doubt they will do. A lot of these servers would end up with very little population as the majority of players progressed. So while you may take your time enjoying Kunark for example, by the time you're ready to progress to Velious, there may be no one on it because everyone else has moved on to Luclin. It would also require a ton of character transfers, which I doubt they will be doing for free.