This is what i think.
Luffy will hear from Cp0 or x drake that sabo has been captured by the navy. Luffy after defeating kaido will negotiate with the navy. He would say “i give you kaido for exchange i want sabo”. Cp0 would accept the deal.
The straw hats go all the way to marineford to get back sabo but the navy trap the straw hats fooling them. This would make Luffy similar to Oden, Roger and Whitebeard. That they all get fooled easily.
Luffy would then free kaido and ask him to help him defeat the navy. Kaido who respects luffy for nearly killing him would accept his deal and fight against the navy. They wont fight straight away but luffy declares a war and that they will rescue Sabo.
Luffy: “This time i wont lose my brother”
Luffy will hear from Cp0 or x drake that sabo has been captured by the navy. Luffy after defeating kaido will negotiate with the navy. He would say “i give you kaido for exchange i want sabo”. Cp0 would accept the deal.
The straw hats go all the way to marineford to get back sabo but the navy trap the straw hats fooling them. This would make Luffy similar to Oden, Roger and Whitebeard. That they all get fooled easily.
Luffy would then free kaido and ask him to help him defeat the navy. Kaido who respects luffy for nearly killing him would accept his deal and fight against the navy. They wont fight straight away but luffy declares a war and that they will rescue Sabo.
Luffy: “This time i wont lose my brother”