I don't think Sephiroth of all characters makes Nintendo reps more likely, lol.
Speaking as someone who's done this before, we gotta stop knee-jerk reacting to reveals and thinking that "[INSERT CHARACTER] is in for sure now!" A new reveal does shake things up, yes, but does Sephiroth automatically mean certain choices are "locks"? Personally, I'd say no: He's put a few pieces back on the board, but one instance of something happening is not a pattern, we cannot conclusively use it as evidence for something being a precedent IMHO.
As for the "Nintendo drops all the big characters first" mentality, while that is typically true, that doesn't mean they purposefully choose characters that they think are "boring" or "not hype": That's a community made mindset and it's completely subjective as well.
I'm with
Scoliosis Jones
on this one: It doesn't matter who the final character is because it'll be viewed as a disappointment anyway. Fandoms who viewed it as their "last shot" will feel burned regardless of who it is because it wasn't their character.
TL;DR: I don't think Sephiroth means every character from here on will be "downhill". People said this with Steve, and then the One Winged Angel showed up.