I think there might be a way to sneak in Sephiroth & Necron Side-Story wise

[Kaliesto]

As we all know almost all the Classic FF 1-9 Villain Reps except two are in the game.

Chaos (Present)

The Emperor (Technically, if you consider Vauthry's 2nd form, there is some similarities)

Cloud of Darkness (both her iterations)

Zeromus (I honestly think Zeromus is currently on the way in later story patches in my opinion, its pretty obvious)

Exdeath (Present)

Kefka (Present)

Sephiroth (Not Present)

Ultimecia (Technically, but given a twist, maybe they can revisit this with a ultimate raid with the Real Ultimecia where the boss digs deeper for more power to assume that form)?

Necron (Not Present)


Now concerning Sephiroth:

I think what can be done to introduce him that would make sense is he can be a leftover data construct Omega didn't use for various reasons. Seems like it would be the only way it would make sense.

Now concerning Necron:

I think he would make a good side-story villain for Ultima Thule, and there is many ways to go about it, and after all Necron represents Death.

Now regarding why I'm not mentioning FFX since some consider part of the golden age of the classic FFs

Well truth be told Idk what you can do with Yu Yevon other than being a Eikon, but personally Jecht/Sin had more presence so I think have someone similar to Jecht that can turn into a Eikon similar to the Final Summon and then Yu Yevon being his 2nd form in savage.

That is all got, I wanted to throw around fun ideas at the devs.

[Enkidoh]

Spoilered for EW spoilers

Firstly, Necron - it's clear that

the Endsinger is effectively a female version of Necron, as she has a similar appearence, is meant to be the epitome of despair, has similar vague motives of "the universe sucks and is wallowing in despair, so I'm just going to tear it all down", and they both have eleventh hour appearences in the story with virtually no explanation beforehand (okay, the Endsinger is an amalgamation of Meteions, but given Meteion's own eleventh hour shoehorning into the storyline, it still fits). And the Endsinger throws planets at you, just like Necron does with his 'Grand Square' attack in FFIX's final battle. So it's pretty clear the Endsinger is meant to reference Necron.

As for Sephiroth, again he's already kind of sort of had a reference, or rather, inspiritation - Nael van Darnus was basically Sephiroth in a suit of armour and wielding a magitek weapon - both have long silver hair (as it turns out), both are high ranking generals/officers in their respective militaries, both went insane and both dropped a meteor on their respective worlds (okay, in Nael's case it was an artificial moon but Dalamud's summoning was called the Meteor Project.). And Nael even had a "walking into the flames" scene like Sephiroth did in FFVII! (Okay that scene was in 1.0, but it was still a reference). And Nael led the Garlean Imperial VIIth Legion...

And Golbez is literally in FFXIV now so it's clear we're gearing up for a fight against him...

[ryouma17]

sephiroth is overrated trash i'd like to keep him out of FF14

[Selsix]

We don't need Sephiroth here, there's already enough weirdos cosplaying the guy.

[MikkoAkure]

Sephiroth actually got name-dropped in 1.0:

Why, if it isn't Sephiroth! Or was it Sefiross? Sethirophx? xXxSephirothxXx? There are so many adventurers with similar names these days, it is so hard to recall them all.

The NPC sadly didn’t survive the Calamity.

The other villains are at least monstrous enough to appear in FFXIV mostly setting neutral and there’s loads of other monsters from other games tossed in. But the problem with Sephiroth is that he’s just a guy.

Omega is dead(ish) and I can’t really see them throwing Sephiroth into the game outside a possible FF7R2 event. Alpha and the Omega pet with him with some of its consciousness remaining after it’s destruction were a very special case. Outside of that, it would be just as awkward if they just had Rikku randomly show up and neither exactly fit.

[Absimiliard]

The odds of Nomura allowing Sephiroth to be put in the game in any form that would allow him to be defeated - or even equalled - are slim to none. Barring the weirdness that was Cloud, Vincent, and Sephiroth having cameos in Ehrgeiz: God Bless the Ring, SE has always been very picky about where and how he gets deployed, even in their own IPs. Nael is probably as close as we'll get unless there's eventually some kind of non-combat cameo, although I suppose "winning" the fight only to find out he A.) wasn't trying and B.) isn't even slightly injured isn't off the table. S'how they handled him in KH, after all. Sora could "beat" him, but he couldn't beat him.