If the battles are straightforward as possible (which I doubt)
- Luffy vs. Kaido (obviously with help before, during)
- Zoro vs King (already near the center stage)
- Sanji vs. Queen (went ahead of Nami/Carrot, so he should be the closest to the center stage if BM really is on her way to the stage.
- Jinbe vs. Jack (very close to the center stage with Robin)
- Nami vs. Black Maria (she was close to Kaido, so perhaps they will encounter each other)
- Brook/Franky vs. Who's Who and Sasaki. (Sasaki being left out while everyone is inside and Who's Who out searching for Yamato makes me think Brook/Franky are going to encounter this pair at some point.)
- Chopper/Carrot vs. Page 1 and Ulti (Usopp and Chopper are behind the Nami/Carrot/BM chase; they could encounter each other)
- Robin vs. Fukurokuju (sort of set up previously with her encounter against the Onibawanshu)
- Usopp vs. Orochi (because this shit is hilarious and coward vs. coward hasn't happened yet for Usopp. Plus Susanoo: Look it up)
This would pretty much cover every major figure with a Strawhat lol, but I see this being a lot more complicated anyway, with Scabbards and other Supernova getting some of the battles above. Oda will likely treat them equally.
This also isnt counting any BM pirates (Yamato vs. Smoothie please) or the Numbers, so the Strawhats do NOT strictly have to fight the upper brass like this, in fact I don't think they will. For example:
Kid vs. Queen is a possiblity so is Killer vs. a F6 like Drake or Who's Who. Inu/Neko vs. Jack is warranted. Technically Sanji and P1 have yet to encounter each other again but I doubt that is his fight. There is a possiblity Sasaki fights Jinbe instead. Little things like this along with the fact that there are 9 Numbers and a wealth of BMP monsters to also have potential matchups (Yamato vs Smoothe, Pero vs. Carrot, Amande vs. Kiku or another samurai, etc)