@Thousand:
He's only proven that Luffy knows what marriage is. He hasn't proven that Luffy establishes any special importance to it. Or that he won't, at least, reluctantly, marry Hancock to make her happy. Especially given how much he owes her.
In order for me to respond to this, I have to first presuppose your fanfiction actually comes true, which I will not do. I will, however, note that a large portion of Hancock's character (and appeal) stems from her naivety, much like Margaret's did. The fact that she THINKS her love for Luffy is reciprocated is what makes the situation funny and enjoyable. We all know Luffy won't return those feelings, so why would Oda go the extra mile of having someone who already thinks she is married learn she isn't, learn what it is, ask for Luffy's hand, have the readers acknowledge it is a loveless and romanceless marriage, and leave Hancock to continue living in the same delusion she had prior to learning what marriage really is? This accomplishes nothing that had not already been established, while tossing out the window one of the more (or only) endearing characteristics Hancock has, her naivety. Again, for such an avid Hancock fan, you sure have an interesting view on her character.
@Thousand:
Since when does marrying someone automatically tie you down? For all you know, by the time Luffy becomes Pirate King, the World Government and Marines could have been destroyed and Hancock can relinquish her title to travel with Luffy.
For all I know. "For all I know" is for all anyone will know when this supposed fan-fiction takes place. Oda isn't going to leave us a note saying, "Don't worry readers, by the time Luffy becomes the Pirate King, the WG will be destroyed. 'Nuff said." Proving my point again about your understanding of Hancock, let's remind ourselves what type of person she is by quoting her: "You betrayed your people and set out to sea!! You left us behind!!" // "Be silent, you kingdom-abandoning traitor!" But of course everyone would forgive her if she left because she's beautiful, right? Or are you suggesting their crews would merge? Or would Oda just leave all of this to our imagination? These are rhetorical questions, by the way…
@Thousand:
Nyon can enlighten her later on what marriage is.
(Just reiterating the above…) Anything but naive misunderstandings wouldn't be "Hancock"; at least, as far as her 'relationship' with Luffy goes. Considering she already assumes she is married, why clear up her misunderstanding so she can formally ask Luffy, so he can leave having accepted to a loveless and romanceless marriage. What purpose does this serve, save wasting a chapter on near-filler material?
@Thousand:
Hancock will definitely return in the future, marriage or no marriage. Just like Bon Kurei.
"Just like Bon Kurei", you say? Bon Kurei went to prison. He was transported further down the Grand Line. Hancock, on the other hand, would be going back to her island. As in, the opposite direction of Fishman Island and the New World? Hey, I'm not saying it won't happen. She could, say, bring her pirate crew along to help Luffy in a battle of some sort later in the series. Is this something definite though? By all means, no. Luffy will be in the New World, she will be at Amazon Lily. Any other involvement, save reactions to Luffy's infamy, is at the moment completely unknown and unlikely.
@Thousand:
He wouldn't be showing her affections for Luffy so much. It's gotten to the point where he needs to resolve it.
It's her character TLC! How do you not see that? She's a woman who wrongfully assumes all men in the world are the spawn of Satan. She doesn't know what the outside world is really like, and is as blind in her love for Luffy as you are for her! That is what changed her character from 'hated bitch enemy' to 'likable endearing ally'. Take that away, of for a HORRIBLE purposeless "resolution", and all we have is an understood one-sided marriage for a bitchy character who will be staying on her island for the foreseeable future.