Given the fact that they already threw the "we can only be attuned to the aether of 3 primals" or whatever the old lore was out the window, I think getting new primals in 7.0 and beyond is a given. The lore behind Phoenix is shaky at best and was only ever elaborated on once in a single interview, and Phoenix isn't all too different from Shiva in that it's a primal who is also a person.

Ultimately, nowadays they think about "Is this addition fun?" first before "Is this addition adhering to lore someone else came up with 8 years ago?"

Leviathan, Shiva, Ramuh, and the other primals like Lakshmi, Susanoo, and the Warring Triad are all on the table. Summoner is a Summoner now, so rather than adding abilities or spells interacting with dots or other systems, they can focus on the core of the job.