Update: Expansion and changes is live now!
The expansion will be coming on 5 November as a USD$5 DLC (Download Content) where you play using Garruk's deck (Expansion is entitled "Garruk's Revenge"). It'll be based on Alara-block so you'll unlock cards from there in the new 4 level campaign.
It's cool that the deck progresses and changes to match Garruk's corruption; predominantly green with Black splashes to having more Black cards in the deck. However, The Escapist reported the expansion's difficulty of the bosses and inability to customise the Garruk deck which reminds me of Hearthstone's Character Class challenges in Naxxramas which was an unpleasant experience to say the least.
The pricing model has also been updated to be clearer and the Premium packs are now unlocked from Multiplayer Mode instead. It has been reported that Wizards of the Coast will plan to reward players that have already spent on Magic 2015. You can get a picture of the pricing model over at IGN. The Premium pack change is available to those who unlock all the original chapters after the 5 November update, source: Gamesutra.
While some over at the reddit sub still wants Two-Headed Giant mode to be added, I think the changes are good and should solve some of the initial complains at launch. The expansion card list is available at Examiner.com so you can start brewing new decks.
