The more I think about it, the less I can see this fight happening. It's like WMMA's version of Pacquaio/Mayweather (ok, that may be a bit of a stretch, but you know what I mean). It'll be talked about, but won't come around. If it took place at 135, then Cyborg would need a feeler fight just to get used to the weight/size difference, then go on to decimate Rousey.
But it'll never come around.