When Kuma was introduced, this plotline was already set into motion.
Oda slowly told us he is King of Sorbet Kingdom, he is a Warlord, he is associated with Vegapunk, he is associated with Dragon & then that he finally lost his Humanity over like around 100 chapters lol.
The Supernovas were last minute creations, that doesn't mean characters like Bonney, Kuma & this entire saga weren't planned.
Yonko were a big part, Oda didn't even know how to handle them because Kaidou & Big Mom were just there as concepts - Oda didn't even plan stuff out. If i have to guess, I'd say he wanted Luffy to be a Yonkou & defeat one but the entirety of the rest wasn't fully planned out.
Oda had Doflamingo though, era of SMILEs & Dressrosa Saga was hinted at pre Timeskip.
And none of this changes the original point = Oda said even in Enies Lobby that 4 Emperors are counterbalanced by Warlords & Marines.
One Emperor & crew alone stood zero chance.