A fight between Luffy and Kuzan would end in 5 secs after Luffy 1 taps the fodder that needed half a yonko crew to sneak attack and jump a old man.
A fight between Luffy and BM is ending in 1-2 minutes after Luffy clowns her and turns her into a amnesiac (again) or worst, worst because BM's childish personality would be extra fitting for G5 Luffy now, who's all about childish personalities and clowning his opponents.
Elbaf will probably be SH's V BBP as a whole, off course Luffy is much stronger then the opposite side and Oda has already shown us that his willing to give Luffy 1v2's so in Elbaf it'll probably be Luffy 1v2ing Blackbeard and Kuzan whilst the rest of the matcheups as as normal 1v1s.
Off course, another theory that I've been cooking is featuring all 3. BM still get's clowned, but she gets Lucci'ed instead, she looks bad in comparison to Luffy solely, basically she gets punched into being a ally (at least if temporarily) and smacks up Kuzan whilst Luffy and BB 1v1.