I formally retract and eat my words from earlier.
Regardless, since we're getting Crobat Prime in the set, looks like there's a good chance that the six primes will be Crobat, Slowking, Ursaring, Raichu, Tyranitar, and Steelix. Since we're also getting Jirachi and Shaymin, as well as the Legends, looks like we're getting some crazy amalgamation of a bunch of Japanese cards; my bet is the remainder of HG/SS, a good number of the Leafeon/Metagross and Battle Starter decks, and just a select few cards from the JP RL set.
Just speculation, of course, so all of you who want to abuse Kingdra Prime can still cross your fingers.