I'm thinking they will meet at the Reverie. It'll be a good time to introduce Dragon, a meeting where the world's leaders will meet. Depending on how Dragon participates in that arc will determine if he and Luffy will become enemies or simply associates. If Dragon attempts to be violent in the likes of King Neptune, Vivi, or the Rikus, then Luffy may have a bad reaction if he doesn't come to understand his actions.