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The new version of forge (05-31) makes 3 additional preconstructed theme/intro decks forgeable, from Zendikar, The Swarm, from Tempest, Time Curse from Starter 1999.

The complete list (34 total, the additions are italicized):

New Phyrexia: Devouring Skies
Mirrodin Besieged event deck: Into the Breach
Mirrodin Besieged: Doom Inevitable
Scars of Mirrodin: Deadspread, Relic Breaker
Rise of the Eldrazi: Invading Spawn
Zendikar: The Adventurers
Magic 2011: Reign of Vampirism
Alara Reborn: Dead Ahead, Eternal Siege, Unnatural Schemes
Conflux: Naya Domain
Shards of Alara: Bant Exalted, Primordial Jund
Master's Edition II: Haze of Spores
10th Edition: Arcanis's Guile, Kamahl's Temper, Molimo's Might
9th Edition: Custom Creatures, World Aflame
Mirrodin: Bait & Bludgeon
8th Edition: Expulsion, Sky Slam, Speed Scorch
7th Edition: Bomber, Infestation
Starter 1999: Deadly Instinct, Time Curse
Portal Three Kingdoms: Shu Kingdom, Wu Kingdom
Portal Second Age: Martial Law, Nature's Assault, Spellweaver
Tempest: The Swarm