Bowsette - Bowsette confesses

And then Mario took her by her hands and…

Swung her in a circle several times before tossing her a great distance and yelling "So long-a, gay Bowser!"

This is the true ending. Having been rivals for so long, they probably know each other better than either of them knows Peach. It's the same way a general who fights another for a long time come to respect him. By now, Bowser must have some sort of appreciation for Mario's perseverance.

Forgetting Peach and just having each other would be the only way to guarantee peace.