Later on in the day, Hancock sat on her now massive throne, thinking about how to go about making Luffy hers. She didn't think she'd be able to convince him to stay on Amazon Lily. Even if she were strong enough to force him, he'd actively resent her for it. And she felt using the exchanger to make him want to stay here instead of following his dream would take away something vital to making Luffy, Luffy. And so she decided to try joining his crew, but at her current size she wouldn't fit on any ship but her own (also resized to fit her).
She supposed she'd cross that bridge when she came to it. For now, she wanted to become useful to her beloved Luffy. At the rate he was training, Hancock feared that he'd surpass her soon even with her enormous new size. She remembered how he managed to defeat both of her sisters at the same time without even knowing how to use Haki. And now he was learning from one of the best. As much as she loved Luffy, the idea of being weaker than him still bothered her. She liked the idea of being the dominant one in the relationship.
Plus, even after all his training there were still people out there who would be much stronger than him. If he was left to deal with them himself she could lose him. She wanted to be able to protect him when that happened. This gave her an idea, though. If she used the exchanger, she could use it to steal the strength from one of his potential strong enemies while making them much weaker. Then, if they faced them, Luffy could come out on top on his own and she could take down the much stronger enemies. And what better target, than the world's strongest creature.
Before she did that though, she'd have to temporarily reduce her size. If she gained Kaido's strength as a regular sized person and then returned to her new giantess stature, she'd be even stronger than Kaido was before the transfer. But, who to give her current size to? It would have to be someone short, so she could gain the most benefit from this. Immediately, Blue Fan came to mind. The idea of trading heights with someone so short disgusted her, but it was for luffy, and it was only temporary.
With a push of the button, she had her height swapped with Blue Fan and she became extremely small by her standards. Hancock was still fuming at having to do this. She might just turn the girl to stone later just to blow off steam. But for now she did what she planned on. With a push of a button, she swapped her physical abilities with that of Kaido's, their strength, speed (Kaido's actually remarkably fast), toughness, pain tolerance, stamina, all of it. Immediately she felt the difference. With a punch to the ground, the entire thing spider-webbed. Before anyone could be shocked, she tried a little experiment swapping the current condition of her floor with that of a random celestial dragon. To her utter glee, it worked.
*Meanwhile* "WHO DID THIS TO MY BEAUTIFUL TILE FLOOR!!!"
As a final adjustment before getting her size back, she swapped her Haki strength and mastery with Kaido's and felt amazed at all the possibilities and level of power she'd previously missed out on. With that, she returned to her new giant size, swapping back with Blue Fan, who she still intended to squash flat out of spite. What else, what else? As she thought about it, it finally hit her how she could join the Straw hat pirates and what preparations to make with that in mind.
Choice 1: She has the Straw hat pirates team up with the Kuja pirates, officially as their subordinates, to both keep the world government off their back and ensure she has a full fighting force at the ready to help them if need be. In this situation, she uses the Exchanger to give Marguerite a similar power up to her own, swapping her physical and Haki abilities with Big mom while giving her Blackbeard's devil fruit power. This way, she cripples another two of Luffy's potential enemies and gives herself additional backup in case she's somewhere else and can't protect Luffy herself. While still being stronger than her.
Choice 2: She waits until she and Luffy reach Sabaody to swap her life and position with that of Nami. The Sunny would be resized to fit Hancock, and she'd gain Nami's navigation and weather skills. In this case, the swap with Big mom and Blackbeard would instead be done with Nami. This way, the Kuja are in safe hands, and Hancock has one less rival for Luffy's affections.
Choice 3: Readers choice.