Yeh it makes sense how Tashigi could actually be Kuina because they do share alot of similarities like appearance, Swordfighting, and even hating on being looked down upon as a woman. I was also thinking that maybe she had gotten Amnesia from falling down the stairs plus Smoker could have discovered her one day and offered her a job in the marines. It's possible but it could be wrong, but knowing Oda you can never really predict alot of things in One piece hmm another thing too like what if when Zoro becomes the greatest swordsman she comes up and wants to challenge him one last time to prove who really is the best swordsman :)
They are very similar and Kuina looking tough was all show but her vulnerability and kindness are identical to Tashigi's (even the same date of birth and "twins" answer seems too logical). In some way, Tashigi tries to act tough as well. In addition, I find it even more strange that Tashigi was fascinated the most by Kuina's sword. I know she is a sword geek but when she fought Zoro she declared that she was gonna take the wado and Zoro said that he will never give it up. Jeez, I mean Oda coincidentally drew her with the same face as Zoro's dead friend. Obviously, he has something big to reveal and we may have to wai the end of the series.
Her amnesia might have something to do with Koshiro helping Dragon which could have caused her a lot of trouble. Moreover, other people on the net have noticed that the Gorosei looks a lot like Koshiro meaning that old man could be his father. I mean the son of one the gorosei helping the worst criminal would be quite shameful and dangerous for their family honor and Kuina (she might be the sole heir to their legacy). Maybe for that reason, the old man staged her death, erased her memory (to cut any ties with the revolutionaries) and encouraged her to become a good peaceful housewife in loguetown which may have pissed her to the point that she became a marine and impressed Smoker (maybe she displayed a great feat of courage and honor). Vegapunk proved it was technically possible to erase someone's memories and replicate a body (Kuma). Maybe that's the reason why Tashigi is here cause maybe Kuina was actually kidnapped and experienced.
@galaxy9000 Pay more attention. In the first picture, Tashigi looks like she is no more than 8 years old. On the other picture, Kuina must be like 10 and 11 cause at the time she met Zoro, she was starting to hit puberty. How does that explain the glasses and the katana book ? Well, we never really got to see how Kuina looked like 2 or 3 years before she met Zoro. Moreover, we have never seen what Kuina did in her spare time, the idea of her reading katana book was far from impossible. Maybe Kuina did not wear glasses when she was fighting (Tashigi seems to prefer fighting without her glasses as well) cause she was afraid that her opponents would look down on her.
@desa Zoro's dream is also for her sake (he is actually very selfless). If Tashigi turns out to be Kuina and accomplishes her dream by defeating him, he would be happy. Moreover, Tashigi also wishes to defeat Zoro as well which is by no means an easy ambition (the pupil of the greatest swordsman) and it could be forshadowing to how the manga will end. I assure you that Kuina would have looked as pathetic as Tashigi if she had never met Zoro (jeez even her own dad never believed in her strenght) cause she would have been stuck with her sexism issues and neglected her training.
Whenever Tashigi and Zoro meet, it always goes back to Kuina. Damn, even the way he defeated her in loguetown was almost identical to the way Kuina won her 2001 match against Zoro. I still wonder why Zoro did that if he thinks Tashigi is not Kuina. Maybe his body knew it was actually her and Zoro craved to defeat Kuina that way as a payback for the 2001 losses in a row. I do find it funny that Zoro and Tashigi's relationship is reversed froom his friendship with Kuina cause he is the one taunting her (in a cute way) that she is weaker than him. I guess he always wanted to say that to Kuina unless its really her.