Next Demi-Primal?

[Flana]

Basically we could only hold or attune to the aetheric signature of three primals - which is why for demis we just channel the aether from the land and by the same token get phoenix (whole of Etheirys doused in bahamut and phoenix mojo juice during the calamity)

Trances were the cheat because you don't manifest an extra being and instead channel the primals power through your body. This is why we got Dreadwyrm Trance back in Heavensward. Then SE forgot about that plot point and just let you summon bahamut with a nonsensical 'grand spell' from Prim in Stormblood which is never explained or even hinted at throughout the entire questline and piled phoenix on top in Shadowbringers without any context whatsoever. Man would have been nice to actually get a Summoner job quest in ShB to explain all of this with some context and not a Stormbood sidequest masquerading as a job quest instead.

Like I said before, SE hasn't cared about any of the foundational lore for Summoner in years. They just do whatever and retcon in a reason for it to exist if we even get that. To be more on topic, a new demi summon isn't likely to happen because it would create a 3-minute loop (unless it replaces bahamut or phoenix and SE will never do that), which is antithetical to their current job design. That said justice for Alexander. Giant robots and laser beams, can't go wrong with that.

Edit:
Also the above theory doesn't hold water considering we got demi-phoenix on the first and have absolutely no problem demi-summoning on the first or the thirteenth. The result of a bad retcon that muddies the water just to provide a shallow spectacle.