Future Primals and potential implementation into lore.

[Seraphor]

Remind me again which other Final Fantasy they were in?

Susano as in Bravely Default/Second and FF Dimensions II.
Ravana was a minor boss in Vagrant Story.

[NewAgeDoom]

Except half of them already are in the game, as regular bosses/monsters (except for Belias as an egi version)

snip.

Ah, I wasn't aware of that. Granted this is my first character, and I'm only Level 60.

A bit disappointing about Belias. First summon in FFXII, and it's a palette swap.

Wouldn't mind seeing other things like Valefor from X/X-2, Atomos from IX.

[Seraphor]

It could also be the 3 personalities of Yotsuyu. Her original self, her sadistic side, and her innocent side.

I like this more to be honest. Yotsuyu struggling to come to terms with multiple personalities, somehow reflected back at her through the mirror.
Lets say she smashes the mirror, into three pieces, and releases the aether contained within it...

[Enkidoh]

Atomos from IX.

Well, again sadly Atomos already appears in the game multiple times, even back in 1.0 (it first appeared in FFXIV 1.0 as a non-killable entity that randomly materialized over aethertyes as part of the Seventh Umbral Era events that led up to 1.0's shutdown, where it drained the aetherytes and turned them orange, and later disgorged very high level voidsent enemies like ahriman and gargoyles that would immediately agro (and quickly kill) anyone who happened to be in the vicinity (not even a party was usually strong enough to take them on).

In ARR Atomos appears as a boss in the Crystal Tower raids, firstly in the Labyrinth of the Ancients, and then later in the World of Darkness. And from what I understand, it's meant to actually be the same Atomos from FFXI (an entity that devours timelines, essentially whole universes.) So alas, a Primal form is out of the question.

There is also a rather cute minion of Atomos you can get too (this one is pink and more resembles it's FFV original).

So basically Atomos is already in the game I'm afraid.

[Okamimaru]

Wouldn't mind seeing other things like Valefor from X/X-2, Atomos from IX.

I believe valefor is a basic monster in game already

[Remedi]

It is but that is because FF3 had a mob named valefor, that said it was mentioned by a group of lalafell in the past as a primal of sort, but we'll see if that story will ever go anywhere

[Eloah]

I'm gonna say this as a funny nitpick, but isn't Valefor female? But then again Levi was always male until XV, so who knows.

[Vidu]

I believe valefor is a basic monster in game already

She is indeed - how ever during the questline with the Doman Adventurers' in Mor Dhona one of the guys you're tracking down attempts to summon Valefor when you catch them.
No primal appears and they flee however.

Since the questline never continued past that, there still is the chance to see Valefor as summon at somepoint since there seem to be some sort of record of her being a primal in Eorzea. (As in: there must have been some sort of description and believe tied to Valefor)
So fingers crossed for that!

[Stellafreyja]

The Magus Sisters could be an interesting Primal fight. I'd love to see that. Or even some of the fal'Cie from the XIII saga. (Orphan and Barthandelus especially)

[Lambdafish]

Susano as in Bravely Default/Second and FF Dimensions II.
Ravana was a minor boss in Vagrant Story.

Of which they are that entity in name only. The ravana and susano-o (not even the same name) from their respective games are completely different, as much as Shiva is completely different from the Hindu god, and bahamut isn't a planet sized turtle