One of the hidden aspects for Destiny 2's move to the Blizzard launcher and Battle.net is that you will be able to purchase the game using Blizzard's Balance system. That means farming gold in World of Warcraft could get you a copy of the game when it comes out on PC as early as September 8.
A Blizzard FAQ connfirmed that you can use the Blizzard Balance system to buy the game, with the World of Warcraft Token system allowing you to turn gold into credit on your Blizzard account. Previously WoW Tokens could only be used to trade gold for game time, or buy a token in exchange for gold on the WoW Auction House. But Blizzard changed the rules a bit in February where tokens can now be exchanged for Blizzard Balance to be used for any Blizzard games. This system will now be used for Destiny 2 as well.
Right now, about 106,000 gold will get you a WoW Token. and a WoW token converts to $15 of Blizzard Balance. If you figure that Destiny 2 will likely run around $60, you'll need four tokens, which means farming about 424,000 gold.
So get the money set aside in your Blizzard Balance now to take advantage of any pre-order bonuses that will be announced as the game gets closer to launch, or any future items that Bungie may put in the Blizzard store down the road.