OK, I kinda feel like I should spend time doing other things after coming up with this, it's so... yeah.
Basically, is a Kuina x Zoro theory. She may not be Tashigi.
This thought came up because I was thinking of who Luffy will end up with and thought it would be somewhat poetic if he married a Marine.
Here's the deal. The revelutionnaries who were on Zoro's island took Kuina and faked her death. Think about it, there's a lot of wierd Devil Fruits and crazy science in One Piece, it could happen. Basically she wants to be the greatest swordsman. The whole thing with her is that she's a girl and thinks she'll get weaker when she grows up. So, Dragon and Ivankov take her away. She works for them and in exchange for her work, Ivankov turns her into a guy. She trains and gets stronger and eventually through some crazy events, she becomes the new shogun of Wano, working under Kaido.
Now we go to the Wano arc. Zoro faces off against the shogun while Luffy and Law fights Kaido. They fight and come to a draw. Kuina reveals who she is. And then, Zoro x Kuina... but under different circumstances... if you catch my drift. Although I guess Zoro might just kill cause fuck it, he wants to be greatest swordsman and ain't got time for that shit.
Oda has done this kind of stuff with the Okamas so not out of the realm of possibility. Of course, this relationship would be end of story stuff since Oda doesn't want to add romance stuff into the story.