During his recent interview with Mirror, Kingdom Hearts Game Director Tetsuya Nomura revealed that the development team is still debating whether or not to bring back Sephiroth as a secret boss battle. Nomura stated..
“I can’t say for certain [if there will be secret bosses]. I hope, hint hint hint. I can’t say for certain. In terms of Sephiroth, do you feel like you want to battle Sephiroth again? (Laughs.)”
“I don’t think I can beat him!(Laughs.) In terms of the secret boss, we can’t say anything at this time, but in terms of Sephiroth specifically, my development team are worried that we’ve had him come back so many times, so they’re maybe worried it might be redundant at this point. We’re still deliberating on it.”