I still think 4 from each seas make sense. Monster, mid and weak quartet is a logical option (Luffy is not included, he is considered as captain)
1- Luffy
2- Zoro, Sanji, Jinbei, Yamato
3- Franky, Brook, Carrot, Robin
4- Nami, Chopper, Usopp, ?
The only thing is how you can fill ? Vivi is an option but she strikes me as way too weak compared to Nami, Chopper and Usopp.
Maybe Carrot can be placed in weak quartet, though in that case we would need a nakama for the middle.