Um, I think you're forgetting Shaymin, Regigias (and the Regis), Latioas, Latias and Heatran. Assuming one doesn't count Manaphy as a legendary.
Except for Undaunted which had three, we've only had two LEGENDS per set. I'd either guess all three Kanto birds or Lake Tiro done in the same style as the dogs, or Giratina and Shaymin/Regigigas and Heatran/Latias and Latios (two of the three). I imagine Arceus would be hard due to his status as 'Mulitype'.
As for Primes, we have plenty of options. I think, over exposed as I think he is, Mewtwo might be one of those Primes. Again except for Triumphant giving us eight of them, all other set have had six primes, so I'm guessing that's what we'd be getting. I wonder if there will be another Alph Litograph in that set.
This is assuming they have Primes and Legends in this set (I would assume so, but you never know).