Guess we shouldn't be surprised that they gave Sephiroth a bigger role in Midgar. He literally makes no physical appearances until the Kalm flashback after you leave. And they needed a bigger set-piece boss fight to end on than Motorball, granted Motorball looked pretty freakin' rad from the trailer.
they're cowards, tbh. Sephiroth was built up really well in the original with the Shinra massacre, and then the next thing we hear about him is that creepy descent into insanity. Its a total waste to blow their load on him this early. Not like this game is going to be hurting for sales either way