I feel Rousey, JJ, and Gadelha are the best three women's fighters P4P in the UFC. Miesha Tate or one of the better strawweights are probably a distant #4.
If Rousey and JJ can stay dominant then maybe we'll actually see the fight. It'd be an interesting new contender for Rousey, and it would be a payday for JJ in a low pressure no-lose fight where she's expected to lose and would keep her belt even if she did.
Stylistically, I'm not sure how they'd match up, because we haven't seen Ronda against a striker like her. None of Ronda's opponents have looked to maintain distance and oustrike Ronda technically. If she fights Holly Holm then that'll no longer be the case. JJ has takedown defense too, but we haven't seen her defend specifically against a judoka (especially one of Rousey's calibre). I'd have to pick Rousey because she has power as well as the takedown threat and ground skills.