I believe Luffy’s dream is to become King of the world and found a Pirate Kingdom based around freedom, since it’s the inversion of Blackbeard - Xebec’s dream. Luffy ending up with a Pirate Empresses who controls her own country that could be the start of this new pirate nation fits with this idea.
In the story there are 5 potential candidates for this, Vivi, Boa, Shirahoshi, Bonnie, and Rebecca. All 5 of these princesses have aligned with Luffy or actively been pirates themselves. The easy route here is to just go with the actual pirate empress Boa Hancock, but given Joy Boy’s strong connection to Lily, former mermaid princess, and Buccaneer races it’s possible Oda would rather have one of them step up as the new pirate empress.