Luffy rival is BB, then if he is his Xebec, nemesis, or whatever then that is true too. Actually it is the best description since Luffy is Roger, Joyboy and the sun, BB is Xebec and the moon. The point is that after Luffy has his eos fight with BB whoever is left to cover the role of new yonko and/or wannabe rivals like Kidd or Law they would just be jokes compared to BB, even worse after considering all the symbolism and closing of the OP story that will happen (ancient kingdom, void century, Im and the WG).