The idea is still under construction, but the match-ups are based on their similarities:



Hashirama vs Whitebeard (both are titled the strongest man/shinobi)

Oonoki vs Blackbeard (Jinton and Earth Jutsu vs Darkness and Earthquake)

Third Raikage vs Kaido ( both are strongest beasts - 3rd Raikage sort of)

Kaguya vs Big Mom ( both are strongest female characters)



Madara vs Shanks - I honestly don't know, I guess both are cool looking. Shanks and Hashirama share similar personality, both are cheerful and easy-going but can be very serious when it's needed, which make them suited as rivals against Madara. Also, Shanks is most likely to be the last Yonko to reveal his true power, so he'd fit better as a final boss like Madara.