(Possible spoiler for Dressrosa arc)
This is my theory based on what I recal from the anime:
So I just started watching the punk hazard arc till episode 770 all over again and I got this feeling like Luffy and Law are like the new generation of Gold Roger and Whitebeard Respectively (only basing this on what I have seen so far from the anime). I'm not completely sure on this but if I remember correctly Roger and Whitebeard were on relatively good terms maybe if even friendly terms and the Strawhat Alliance founders Law and Luffy are both on friendly terms too.
So what do you guys think?
[Disclaimer] Yes I'm a airhead and figure things out slowly >.<