LEAK: Standard Showdown packs for Ixalan will be similar to Yugioh's Tournament Pack series. This will be where we will see Masterpieces from now on, and each set will have 2 Masterpieces. Each pack will contain 5 cards: 2 Commons, 1 Uncommon, 1 Rare or Mythic, and 1 Foil, which could either be a foil version of a card in the main set, or a Masterpiece. All cards in the set except the Masterpieces will be Standard Legal.
Here is the set breakdown for the First of these Standard Showdown packs, coming in October of this year:
Masterpieces:
Tarmogoyf
Lightning Bolt
Mythics (5)
Nissa Revane
Jace Beleren
Gideon Jura
Chandra Nalaar
Liliana Vess
All with errata to "Legendary Planeswalker"
Rares (9)
Chandra's Phoenix
Weathered Wayfarer
Amulet of Vigor
Windborn Muse
Lavaborn Muse
Seedborn Muse
Dreamborn Muse
Graveborn Muse
Allosaurus Rider (with errata to "Dinosaur Elf Warrior")
Commons and uncommons will be spoiled later on. There are 10 commons and 10 uncommon, making this a 36-card set.