This is the perfect occasion for a group fight and to get the New Generation name out there as the strongest pirates. It's been clear from the start that is the set up
Also Luffy is no where near beating Kaido 1v1 if he does IMO that is bad writing. Luffy beats Kaido 1v1 he is the strongest in the world and is PK. There is not point to having a 1v1 vs Shanks or BB which is also what makes this group fight IMO relevant as well.
Kaido will probably get some nerf from this 5vs1 and then it will devolve into a 1vs1 with Wano on Luffy's back as Kinemon has stated. Luffy has stood in front of Kaido and told him he will be PK, if he only gets through this by gang tactics throughout the whole fight I will be surprised.
BM will be the next to go down, as Luffy has already claimed her after he takes down Kaido, do you think he is going to group fight her as well- if so what is the point? He would have already proven he can take down a yonko 5vs1, if not you fall into the same b.s. trap of "it's too soon for Luffy to be this strong".
BB can negate the prowess of g4 and provide a suitable nerf.
Shanks is equal or stronger than Kaido from Marineford portrayal and his better haki mastery might provide a more difficult matchup and I don't see a serious arc based fight being held against Shanks who Luffy loves and is seemingly a good guy. A "rival" fight doesn't have to be seriously power scaled, because Luffy won't need to be in growth underdog mode.
Akainu has had his haki bloom and has fought whitebeard, he can be as strong as Oda wants him to be.
Imu is the king of the world.