I'm willing to bet a clean 20 bucks that Sephiroth's victory theme is gonna be the "Estuans Interius, Ira Vehementi,
Se-phi-roth!" part of One Winged Angel.
Anyways, we've got precedent for Ultimate giving characters unique nonsense in their victory screens like Simon's post flash whipping, Joker's UI changes, and Piranha Plant's additional character model with Mario getting bitten.
Considering Sephiroth already broke precedent by coming with a change to Cloud's final smash, wouldn't be hard to imagine Sakurai going the extra mile and adding a few flame effects or flame .pngs for a single victory screen.
I'd say as far as victory poses go, I'm gonna bet the 3 we get are
1) The classic slash, slash, backhand masamune victory pose for when he's in your party
2) The Nibelheim incident as mentioned above and already theorized, basically a turn towards the camera and a menacing walk away,
3) Sephiroth stabbing the ground and then T-posing. Spoilers if I explain this one, but you already know what it is.