Let me give me thoughts on this since I just joined this forum. This is all pretty much what I expected, except the part of Garp and Roger allying.
My guess since Whole Cake Island for Kaido's bounty was... 4.5 billion. It's 4.6. Why? Because Luffy's bounty has a pattern of tripling.
Alabasta: 30 million to 100 million. (Slightly more than 3x)
Ennies Lobby: 100 million to 300 million. (3x)
Marineford/Dressrosa was the exception, since 300 million was the "limit". So he slowly went 300 -> 400 -> 500.
Whole Cake Island: 500 million to 1.5 billion. (3x)
So Kaido's bounty is slightly more than 3x current Luffy's bounty, and if it follows Alabasta in terms of fighting (gets owned in 1st fight and comes back to win), Luffy's bounty will be 5 billion, which was my guess too. And another reason why I believe this, is because Whitebeard is 5.046 billion, so only Whitebeard and Roger would symbolically be above Luffy as he becomes a fully fledged Yonko.
Ever since Big Mom set sail for Wano, it became clear Rox would be revived, and Luffy taking down Kaido = Garp taking down Rox. Oda clearly said this would make Marineford look like a joke, and marines are clearly going to join as per Sengoku's statement about Oden.
As for the matchups, I think it was something like:
Garp vs. Rox
Roger vs. Whitebeard (the infamous stalemate)
Rayleigh vs. Linlin