I think Luffy will randomly pass his father while trying to get to someplace and be all, "Hey, old man, did you see where [insert name of enemy or nakama that he was chasing here] went?" Dragon answers him, "Okay, thanks! "
And then 100 chapters later they meet again in some all too dangerous situation and Dragon reveals himself to be Luffy's father. "Whaaaaah?! That was YOU?!"
There are endless possibilities.
Ace's lost half sister
they have met....
but if they meet again... i think luffy's crew would tell him, since i doubt luffy would recognize dragon.