We've misheard the lyrics to One-Winged Angel at some point in our lives.
Chance: 5%. Something tells me that Dragon Quest is approaching lock status, and thus Sephiroth has a massive hurdle to overcome. However, if we pretend leaks don't exist, Jenova's little boy* is probably the most famous person Square owns besides Mr. Strife. And that's always a plus. Aside from that, he gets the boost of Squeenix character since it's been leaked.
Want: 25%. He's not Arle competition, but aside from that I can't think of any good reason to want him. He looks cool, but honestly if I wanted somebody else from Final Fantasy, I'd take Rydia first, and if it had to be from 7, I'd rather have Barret, Tifa, Aerith, or even Cait Sith (in fact, any of the party members would probably be more interesting and easier to balance). I have played his game, and beaten it and him, but I would not say that Sephiroth was the most integral part of my experience, it was probably Cloud and Cait Sith (don't ask why I used this guy or how he ended up being the party's death machine). He also would open the door for good on second Sonic reps, which are right now my worst-case-scenario (guess why? Albeit, I think Tails and Eggman would be very interesting, but I can't pretend they're not Arle competition, and the next time a Sonic character gets rated I'll explain in greater detail), but that's neither here-nor-there and I won't dock Sephiroth on that.
Ryu Hayabusa x 5, I don't have enough knowledge on SNK to be bold with this but I think a flat 13% should be good.