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

[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...