Luffy is really the only one who has motivation to fight both Kaido and Big Mom, but I think the specifics of Law’s plan from Punk Hazard (and all the other moments of buildup since then) kinda necessitates him facing Kaido. As far as the others...
Zoro - definitely Kaido, he has the only sword that ever scarred the guy in the past and his likely (and confirmed) connections to Wano means he’s gotta play a part in slaying the beast (#ZKK)
Law - Kaido, he came up with this plan and has no direct quarrels with Linlin
Kid - Big Mom, he’s already entered her territory and fucked with her in the past
Killer - Big Mom, follows his captain even though they both suffered indignities under Kaido