After watching Morj's "Luffy v Jack" video, I now think this matchup is a real possibility for Luffy's warm up fight before Kaidou.
Like the video too
Im still believe Jack gonna face the dukes but he probably first get defeated by Luffy. If my theory is right and the war gonna take days/weeks, Jack comeback against the Dukes make then. In that case I can see a fast win of Luffy or at least without much problems as a mid diff or lower end of high diff(probably even less).
Remember when Rayleigh show Luffy Advanced CoA? It was against a elefant and now Luffy having himself Advanced CoA to showcase his new ability against a mammut.
In that case I can know even see why Oda pushing up Sasaki and Who'Who to Calamity lvl, probably one of them take the spot for Jack position who gonna lose his. With this I think one of them get enough hype soon and really good feats. Since I believe that Who'Who facing Zoro soon and taking his lose, I think Sasaki gonna be the one who take the calamity position. With this I can see Oda gonna hype a lot Sasaki in the next chapters to making him fit as Calamity opponent.
First battles:
Luffy vs Jack(Lose for Jack)
Zoro vs Who'Who(Lose for Who' Who)
Alliance member vs Sasaki(Sasaki winning)
Second round
Zoro(+x ?) Vs King
Sanji(+x ?) Vs Queen
Jinbe(+x ?) Vs Sasaki
Inu and Neko vs Jack
I expect for Sasaki to having a bounty range of 800M-1.1B Bounty.