Well, Blizzard is seemingly letting all of the beans spill out before any of its BlizzCon conferences take place this weekend!
According to an image straight from Battle.net (which has been taken down), the World of Warcraft: Legion expansion will be due "on or before" September 2016. If you pre-order you'll instantly gain access to a level 100 boost, as well as 7-days of early access for the Demon Hunter class. It's priced at $59.99, or $69.99 if you spring for the Digital Deluxe Edition.
There seem to be bonuses in tow for World of Warcraft, Starcraft II, Diablo III, and Heroes of the Storm. A Hearthstone extra is strangely absent. Expect more info to be officially unveiled at BlizzCon.