But try to think of the system as a whole, using the known constraints you've pointed out. The most notable constraint in my eyes is the new player one, where you MUST level in BFA. How does this work? Obviously there's any number of ways this could be done. Even if you keep players from entering expansion areas, you still have to deal with vanilla zones. They need access to the capital cities. How do you reconcile the fact that they're not allowed to level in vanilla, but still need access to it?
A quote from the link you posted says the following:Instead of assuming that "Chromie time" applies to all leveling areas all the time, let's assume that you choose an expansion. That expansion then exists as "Chromie time" and everywhere else is the main timeline. If you're an experienced player, you can switch your choice, but it's still a choice and you can only have one active at a time. If you're a new player, your choice is made for you (BFA).Actually, to be clear, it'll be scaled from 10 to 50 while you're levelling. However, that's where Chromie comes in - Chromie Time. As you're doing it from 10 to 50, it's Chromie time, it's scaled. Once you hit 50, Chromie will show up again and say, 'Hey, it looks like you're done, cool - let's bring you back to the main timeline,' and then you'll be 50 and Pandaria will have been scaled down to around, I'm going to guess, 35-ish in the new scheme.
Could I be wrong about this? Yeah, absolutely. But doing it this way solves a lot of problems. It prevents new players from leveling to 50 in vanilla without restricting access to the areas themselves. It also allows sub-level max level players to still farm old content (current 110 Demon Hunters are popular due to legion legendaries).
I suspect that you're right that Blizzard will use the current brackets, although it's by no means required. Just because you could level in non-chromie time doesn't mean you should. I don't think anyone would voluntarily put themselves through that except maybe to prove it was possible. Thus the new system doesn't actually have to line up with the old one. No one is going to complain that their character who was 66 leveling in Wrath is now underleveled for for the main timeline Wrath content, because they're immediately going to activate Chromie time since it's significantly faster/more efficient.
When 9.0 hits, what's going to happen to our characters that are currently leveling? Presumably we're all going to have Chromie standing in front of us to explain what's going on (I suspect the same thing that would happen when a character dings 10). Remember many players don't visit news sites at all, so Blizzard shouldn't just dump them into the new system without explanation. Thus, it doesn't actually matter where you are now since the first thing you're going to do is activate the new system (or have it already activated for you when you log in).