My theory is that the next Primal is Exodus, the Judge-Sal, and that brief cameo by Belias in the Summoner questline will finally be justified.
Why? Some of the Final Fantasy XII Espers were based on past final bosses; Chaos, Mateus, and Zeromus all speak for themselves, but Exodus was in fact based on ExDeath, Final Fantasy Vs big bad. The Rising event mentions "the next primal" perhaps being "the last boss", and we already have evidence for the Ivalice Espers with Belias and the Summoner questline, so Chaos/Mateus/Zeromus/Exodus all seem likely to me. They were all "last bosses", and if Belias is confirmed, they should all be showing up eventually anyway.
Add to that the completely forgotten plot point regarding the Students of Baldesion, the vanishing Isle of Val, and Minfilias friend (the sole survivor) Krile, all a fairly obvious reference to Final Fantasy Vs Krile Mayer Baldesion, who we hopefully get before Minfilia as a (hopefully) more capable replacement. If the Twelveswood isn't a massive reference to the Forest of Moore at some point, I'm gonna be sad... Of course given that post credits reveal of the Warrior of Darkness, or more specifically where that scene too place, I guess Zeromus could also be a good candidate, but the Gordian Helm of Fending screams ExDeath at me (and Living Liquid was an obvious reference to Vs Living Flame boss), so I'm going with that...
More likely we'll just see Thordan (Extreme), but that's such a boring thing to speculate~ I'm also not sure if it even works, wouldn't that be another Ultima Ballad style thing, rather than an (Extreme)? I'm fairly sure we always get a new Primal anyway, when we got the Ultima Ballad and Garuda/Titan/Ifrit (Extreme), we also got Good King Moggle Mog, so I'd expect Thordan and something new anyway... It's been two years since they teased me with Belias in the Summoner questline, it's about time they delivered us those Espers... Next logical set of options now that the main six have been done, too. We have to be moving toward Zodiark sooner or later, right?