Luffy is inhering Roger's will and Zoro Ryuma's. The answer is Law.
This entire thing of taking down Kaido was his idea. It's not a coincidence his right hand is Bepo, a
mink.
These guys are Oden's brothers, and when Inu/Neko escaped, they surely let Zou of what happened in Wano.
This wasn't about Doflamingo alone though. He knew Kaido's territory, what his army was composed of, and broke down the odds to 30%. He'd done a lot of research about Kaido, and told Doffy he "broke the gears" of the old era.
The whole adventure started off when Law cut down Kinemon, and trapped him until the Straw Hats arrived:
And then he gathered all the Samurai under his command.
Remember, Oden didn't want the Scabbards following him at first either:
By fate again, they ended following him even in Wano
None of this would have happened with Law. Defeating Kaido was always his idea, and I think Shinobu doubting his loyalty will play a HUGE role later:
This is similar to people doubting Oden as the saviour as well, and Shinobu was the one who quenched the doubt.
Now she's in the position who is doubting the man who concocted the plan to beat Kaido, and the guy is currently commanding the Scabbards.