Even if Cyborg can't make 135, there are a number of marketable matchups out there for Ronda. There's the rematch with Miesha Tate, a potential "Battle of the Olympians" with Sara McMann, fights with fellow submission specialists in Shayna Baszler and Marloes Coenen, and maybe even Cat Zingano (if Cat can stay undefeated). Throw in a fight with someone like Alexis Davis, and Ronda will be set for quite some time.
In regards to a potential matchup between Rousey and Carano, I think Ronda would embarrass Gina. Unless Gina caught Ronda coming in, I'd imagine Ronda immediately initiates the clinch, tosses Gina to the mat, and submits her. Never mind the fact that Gina would be coming off a huge layoff and has had issues making weight in the past.