Perhaps more excitingly, digital pre-purchases for the new expansion will begin "Soon™", and will award you a free 90 at the time of purchase, along with your pet and mount if you opt for the Digital Deluxe version. So you could be boosting one of your characters to level 90 Soon™, rather than at the launch of the expansion.
What's more, this blog also notes that following the collection of player feedback, the option to purchase additional 90s from the store is going into testing. While this isn't new information, or concrete confirmation that this feature absolutely will be added to the game, it is notable that they are testing the feature. There will be testing for it anyhow as Asian realms don't buy expansion boxes, but we could well be seeing it in the West too. There is no mention of price for any of this, not for the expansion, nor for the character upgrade, at this point.