Future Primals: Speculation
With Ramuh and Shiva on their way, we have the six basic elements covered. So where do we go after this? I have a few ideas for later additions...
Asura: People call for randomness in fights? This is the place to do it. Her three heads each have a different ability, and she will use one of them at random. Alternatively, she always covers the entire battlefield across three different AoE attacks, and the tanks, dps and healers each need to eat a different one, so players need to keep track of which direction her heads are facing.
Fenrir: I think this would work best as a roaming Primal, like Odin.
Phoenix: Another roaming Primal. However, when slain, it's corpse remains behind where it fell until it rises again.
Alexander: A good candidate for an Imperial Primal, perhaps an artificial Primal created through Magitek.
There are others that could make appearances but I couldn't think of anything specific for, like Knights of the Round, Eden, Maduin or a Chocobo Primal (a white chocobo maybe, like in 12 and 13?). There's also room for the Ivalice summons, considering the existing presence of Belias and Zodiark.
Any other ideas?