Big Mom and Kaido were intended to be Paradise villains, it's clear from how things were set up.
Then Warlords came into play and things got complicated
Who knows if Big Mom and Kaido were even the Original two yonkos Oda had in mind back then
Oda is insisting on completing his original plan for the manga
I doubt any Emperor was intended as Paradise villain and I havent heard of such concept from Oda either.
We know for sure that initially, Buggy was planned to be Buggy since he is literally among first pirates SHs encountered.
He is the first one to encounter the 3 triggers of the story.
Maybe his journey was always intended to be from nobody to Emperor and possibly more.
If that is the case, then Kaido and Big Mom were optional addition to the story.
However, story of beating 4 Emperors means that Luffy isnt an Emperor so one spot is still free.
Without Luffy, there is a spot for either Kaido or Big Mom and I think Kaido was in the original plan.
The whole post-TS was practically a build-up towards him, however, his end is where it is suspicious.
It is like he never existed. No reaction from the world whatsoever...
Maybe Mihawk was intended as an Emperor initially.
However, in the actual manga, he ended up with a personality that isnt interested in such.
Probably, at this point, even Oda doesnt know which way to go and is winging the chapters as they come.
Ending is probably the same as initially planned but everything in between ended up changing.