The only primal I really wanted to see was the Ark, and it was already included in the game as the Cruise Chaser. I really like mecha, so the Ark was my favorite summon thanks to that and it's awesomeness as a summon. It being reduced to the penultimate boss of Alexander's raid series was a bit disappointing, but at least it was put into the game.

As for what could be put in later on...? Who knows.

Suzaku and Seiryu are fair bets, as we need to round out the Four Lords, and Kohryu is a reasonable bet due to being tied so strongly to the story and the other four. Beyond that one can only guess.

Me? Well, I'd like the original Four Fiends... and perhaps Chaos, with a phase transition into his Dissidia Feral Chaos state. Beyond that I'm content to just roll with the punches, and as they've included past bosses / villains as well as original creations, it's anyone's game.