(FFVII-Remake Spoilers) Nomura, you absolute madman

After defeating Motor Ball, Sephiroth descends from the sky and has a conversation with the party, where Aerith tries to rebuke him. He ignores her and then opens a Kingdom Hearts style portal to darkness before leaving through it. Aerith then changes said portal to the Door to Light from Kingdom Hearts II, and tells the party that they need to go through and change things even though it might change themselves.

There's a flashback to Zack's final battle in Crisis Core, though the scale didn't look quite as huge.

Cloud then fights a giant Darkside (yes, the Kingdom Hearts Heartless) made up of several of the dementors that were around the whole game. After defeating them, the timeline apparently changes and Zack somehow survives his final battle.