Maybe Loki does actually join Luffy's crew at the end of this arc, but Blackbeard defeats Luffy in a davy back fight for Loki.
We could have Blackbeard cancelling out Luffy's nika powers and beating him, Kuzan beating Loki, Shiryu beating Zoro due to his lack of efficiency in obs haki, Sanji losing to burgess. These wouldn't be actual full blown duels but under the ruleset of a Davy Back fight and the BB pirates taking advantage of their gimmicks in them, making clear the weaknesses the strawhats have to work on, for when they have their real fight with the blackbeard pirates.