Bartholomew says he and Luffy go way back.
And unless he is a completely new character we've never seen before, there is no other contenders for a character we've met that "goes way back" with Luffy.
Anyways what do you guys think?
It would be pretty cool if you ask me, they would both be fighting for Ace's Mera Mera fruit to prevent it getting into the wrong hands.
Edit: woops, messed up Bartolomeo's name in the title