During an interview with Disney Infinity TV, John Vignocchi revealed that the Toy Box Game Discs will be sold separately in packs of two this holiday season.
He states that there will be two packs released. One pack will have the two Marvel Toy Box Game Discs (Attack on Asgard, Escape from the Kyln) will be available in one pack. The Disney Original Toy Box Game Discs (Stitch’s Tropical Rescue, Brave Forest Siege) will be released in another.
So there you have it. Essentially you can purchase anything and everything (minus a disney infinity base) separately from the Starter Pack(s). So nothing will be exclusive to any starter pack.