Oda wants to turn the last Saga of One Piece into a romantic fairy tale with the brave Luffy saving his beautiful empress Boa from marrying the dark lord BB.
Oda: This Is Entertainment.
This is the catalyst that takes Koby from a weaker character to a powerhouse. We're being led to believe that, with Boa being kidnapped, Luffy will save her. How? He has no idea where she is, and no intelligence network to locate her.
Koby is going to lose, Blackbeard will steal Boa, and Koby will use the frustration from that loss to become stronger, and hunt Blackbeard down. This is the set-up for Blackbeard vs Luffy and Koby mirroring Rocks vs Roger and Garp.
Boa is the element that will link Luffy and Koby together again. They will both want to save her, since they're both best bois, and, in the process, Boa will fall for Koby.
Still want to see Boa and Nami fight for Luffy, though.