I honestly can't see
any upside for Rousey in fighting Cyborg - at any weight.
The argument for it way back in the day was that Rousey was destroying all these lesser fighters, she'd cleaned out her division and nobody was going to be interested in her fights unless she fought Cyborg. Then Rousey sold about a million PPVs fighting Bethe Correia of all people, confirming that she can make bank regardless of who she's fighting.
...then she lost to Holly Holm, and Miesha Tate has held the belt in the interim as well. Pena has come up as a credible title challenger in the meantime, as has Valentina Shevchenko.
Put simply there's now probably half a dozen interesting matchups for Rousey at 135, all of which will sell big because she's involved, and all of which are much more winnable than a fight with Cyborg at 140 or 145.
The UFC adding a women's 145 division would be a joke BTW. I wouldn't be against them doing it
just to put a belt on Cyborg and keep her busy I guess, but it'd be pretty transparent that would be the only reason for them doing it. It'd be a division that'd barely have 10 fights a year.
I mean just look at this power ranking of women's "featherweights":
http://www.fightmatrix.com/mma-ranks/womens-featheweight/
First of all, they have to classify "featherweights" as fighters at 140 or above just to have enough warm bodies to make the list. So, for example, it includes Gabi Garcia, who weighed in at
245 for her most recent bout. The #2 fighter on the list is Alexis Dufresne, who washed out of the UFC in comprehensive fashion, looking like she had no business being in a cage with any legit professional: Cyborg will
murder her. There are 2-0, 2-1 and 4-4 fighters on that list. Women's featherweight just isn't a competitive division.
So Rousey-Cyborg would be a fight that has huge upside for Cyborg - it'd be easily the biggest payday of Cyborg's career. But it's not a fight Rousey has any reason to take.