I have a theory that Luffy’s grandmother/Dragon’s mother is from Wano.
Firstly, we know that a group of Wano citizens immigrated to the East Blue 55 years ago, including Shimotsuki Kozaburo (Koushiro’s father and Kuina’s grandfather) who founded Shimotsuki Village.
We know that Shimotsuki Village is very close to Goa Kingdom/Foosha Village (where Dragon confirmed he was born when he first met Sabo) because Dragon went there for supplies to help the victims of the Gray Terminal fire. We also know that Dragon is 55 years old, meaning he was born the same year the Wano immigrants came to the East Blue. When Oda is specific with years like that I believe it means something important.
Secondly, Dragon’s tattoo is very similar to the symbol seen on Shimotsuki Ryuma’s corpse’s kimono which also features the kanji of the word “dragon”. The timeline matches up that a woman from Wano could meet Monkey D. Garp and have a child 55 years ago (Dragon’s birthday is October 5th meaning he was most likely conceived that same year), the child would be named Dragon after the importance of dragons in Wano culture (Ryuma’s infamy as a dragon slayer).
The future Revolutionary Army leader would have Wano blood, the only nation not affiliated with the World Government, and would eventually get a tattoo in honor of his heritage.
Lastly, we know that if Luffy was in our world, his ethnicity would be Brazilian. Brazil has one of the largest historical Japanese immigrant populations, there are over 2 million Brazilians of Japanese descent, and this would be a cool nod to that.
Theory by sinirishcream.