Think of it this way. There was ALOT of talk of a GSP vs Anderson Silva superfight. There was never, ever talk about Anderson Silva going down to WW to fight GSP. It was ALWAYS GSP going up to Middleweight to fight Anderson Silva. Why? Because for one, Anderson couldn't make WW, and for two, GSP was beating everyone at WW.
If you clean out your division you move to a different division. Thats the way it is in boxing too. That is why people are talking about Jon Jones going to HW. Thats why people talk about fights like Conor McGregor vs a LW champ. Thats why people ask about Weidman going up to fight Jon Jones (this came up recently and both fighters said they would do it). Look at how many divisions fighters like Pac or Mayweather have been in. It's hard to find a fighter who hasn't fought in multiple divisions if they were a dominate champion.
Like I said though, there is no 145 div in the UFC for women's mma. If she is coming in she has to challenge Rousey at Rousey's weight.