While some people are still saying Denjiro is Kyoshiro – I never saw how that could be unless the answer is gigantification, plastic surgery and a memory spell on everyone in Wano who knew Denjiro – I noticed a more striking resemblance between Denjiro and another character who’s already been tied to various Wano parentage theories: Tashigi.
If we compare Denjiro to Tashigi when Smoker was swapped into her body, with her hair tied back and a more “masculine” attitude, I think the similarity is noticeable:
Their hairlines, noses, eyebrows and profiles look very similar. They both wear glasses (thought it’s unconfirmed whether Denjiro needs them) and they even have the same hair highlights, implying Denjiro might have black hair like Tashigi.
Add this to Denjiro’s resemblance to Koshiro (Zoro’s master) and the family tree takes shape: Denjiro could be Tashigi’s father, Koshiro’s brother, and Shimotsuki Kouzaburo’s son.
That would explain why Tashigi and Kuina look identical, have the same blood type, and are both named after the same kind of birds.
The only thing that would oppose this theory is that Tashigi is 23, and her Vivre Card says she was born in the East Blue. And as far as we know, Denjiro was still in Wano fighting Kaido 20 years ago. However, he didn’t need to have been present at her birth: maybe 3 years before Kaido took over, when they felt the impending doom, Denjiro sent his pregnant wife away to his family who had left Wano for the East Blue three decades before (we know Kouzaburou left Wano illegally more than 50 years ago).
But perhaps she never reached them, died not long after giving birth and Tashigi was raised in an orphanage and was recruited by the Marines like many children are. With only her Meito as a connection to her origins (remember the Ringo practice of giving children a sword at birth?), that’s how she became interested in swords from an early age.
*Theory by akazaya9