In my headcanon, Luffy and Nami are biological siblings, and they're also Whitebeard's grandchildren. After Luffy becomes King of the Pirates, Zoro and Nami get together. Their daughter transmigrates to Marineford and saves Ace. The daughter of Zoro and Nami is from One Piece: What If We Knew the Future, where Zoro, Nami, and the others watched the screen and learned about the future. She transmigrates to a future where the screen never appeared, and Ace and Whitebeard are destined to die.
Theme: Ace and Whitebeard don't die
Outrageous plot, but fun to watch