Legacy of the Void, the next expansion for StarCraft II, is now available for prepurchase. The expansion, which includes new units for all the races, new multiplayer features, and a new singleplayer campaign, costs $39.99 USD for the standard edition. The digital deluxe edition, which costs $59.99 USD, includes in addition: new portraits and Protoss unit skin for StarCraft II, a pet for World of Warcraft, a card back for Hearthstone, transmogs for Diablo III, and a Voidseeker mount for Heroes of the Storm.
The shop page for the game does not state a specific release date for the game, though it does say "Expected Game Release Winter 2015/2016*" and "*Game is due for release before March 20, 2016.", which means that we can expect the game by late March 2016 at the latest. There is also confirmation of an epilogue past the end of the singleplayer campaign which shows the various races and what happens to them.
After you play through the Prologue and the Protoss single-player campaign, wrap up the StarCraft II trilogy storyline by finding out what happens to each race in the Epilogue.
For the Blizzard store page for Legacy of the Void, click here. Also, for the news post of the start of prepurchases, which has an FAQ and some announcements, click here.
Battle.net - Legacy of the Void™ Now Available for Pre-Purchase
Battle.net - Legacy of the Void™ Shop Page