probably Garchomp Lv. X (alternate art), Snorlax Lv. X, and Porygon Lv. X (alternate art) or Drapion Lv. X (alternate art) or even Palkia Lv. X (alternate art 2) or Mismagius GL Lv. X (since this one later is an SP and we are in the SP era, it might be a good guess)... or they can make their exclusive art lv. x's: imagine Charizard G Lv. X, Garchomp C Lv. X and Rayquaza C Lv. X on the tins... that would be awesome...
I think the chances are great that it will be Palkia LV X Garchomp LV X and Drapion LV X. They feel more fitting because they are all different types and have yet to be released
I'm pretty sure that if they had a Darkrai G Lv X, and if it were coming out for this set, Japan would have already had it, and WPM would put it on the front page =P.