Edit: Theory was made when chapter 716 was the latest One Piece chapter. After chapter 719 it was revealed what Chinjao’s “precious fortune” was. It was worth a try to theorise was it was beforehand, but my theory missed completely. Oh Oda-sensei, nice surprise you had for us with Chinjao’s flashbacks.
It was interesting to learn about this “precious fortune” Garp stole from Chinjao and how along with his power, the fond memories of his golden years were also stolen. More then Chinjao wanting to beat Luffy up in place of Garp, he wants Garp to feel how it feels to lose something precious to a person. It could very well be that Garp and Chinjao were friends at one point and this “precious fortune” Chinjao could be talking about is his little sister. Could Garp have captured the heart of Chinjao’s younger sister and “stolen” her away from Chinjao who thought of her as someone very precious? Having his little sister fall in love with Garp could have soured Chinjao’s memories from then on which led to Chinjao feeling the ‘fond memories of his golden years’ were stolen by Garp. As a result of this bitterness, Chinjao could have challenged Garp and consequently got beaten to a point close to death which only added to this bitterness and hatred Chinjao was feeling. Chinjao’s sister could very well be Luffy’s grandmother therefore making Chinjao Luffy’s granduncle O.o. Now why would a granduncle (Chinjao) want to kill their grandnephew (Luffy)? On top the immense ill will towards Garp, I suppose it is also the same reason for why Garp beat Ace and Luffy to a point close to death when they talked about being pirates, he just didn’t like what he heard. This similarity between Garp and Chinjao could be what attracted Chinjao’s younger sister to Garp, along with Garp’s natural charismatic side =D. Chinjao already has the Haoshoku Haki, so I wouldn’t be surprised if Luffy and he are related in some way =P. One of the requirements/principles of the Hasshoken style of needing to shed any and all emotional attachment a user has with their loved ones could have some connection to the reaction Chinjao had towards Garp (was that principle put in place because of Chinjao?). Also it could just be nothing but just like how Luffy, Dragon and Garp have marks on the left side of their face to signify similarities between those characters, the “12” tattoo on Chinjao’s left side of his face may also represent such a meaning and indicate that he is in someway related to Luffy =/. I could be horribly off the mark with my interpretation of what Chinjao said, but within my head, this does sound like a possibility O.o.