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.