Oh, I see. That chest will contain the power to defeat Yellow Diamond because Rose knew, in her infinite wisdom, this day would come.

When she shows up, Peridot will suddenly betray the Gems and join her for reasons that don't make sense and are stupid. But then, Yellow Diamond betrays her but is instantly forgiven by Steven. She then gives him the key, and it turns out to be magic and it opens the chest. Steven then powers up and a bunch of multi-colored gems grow around his body in a big mess and he one-shots Yellow Diamond.

Then Steven becomes a Princess, or something…

No,no,no, Steven becomes a princess first because the toy company that's going to make toys for this show wants it to happen on the show so they can sell a $50 piece of plastic.