Law and KidD are most definitely the major rivals of Luffy as they can fill in being the next WB/Shiki and thus have higher bounties than them. Doubt Luffy only rival is BB and he won't be around for long since he is next Xebec. Like Roger it's stupid to think Luffy only have one rival. There be those to challenge to the extreme like the same for Roger to Whitebeard, Shiki, Garp, Sengoku, Xebec, and so on.
Luffy is gaining his own as shown in the story as next generation who be top with Luffy looks to be Kidd, Law, Blackbeard, Smoker, and another (Debating who be next Sengoku. Maybe Hina since her and Smoker are close or someone else?).
It's not farfetched as Oda is pushing Law's importance for a long time and stupid to think he won't get stronger and be major threat not just to Luffy, but to entire world like WB was. That goes for Kidd as well.