Typically you can spot future challengers years out from fighting for the title. I saw potential in Nunes when she ko'd Julia Budd in Strikeforce and that was like five years ago. Unless there is an unknown female fighter out there who is planning on setting the world on fire and making it to a UFC title shot within the next 18 months, these new challengers just don't exist.
You do realize that Rousey beat Tate in less than a year after her first pro MMA fight? And 19 months after her amateur debut?
Rousey vs Munoz - August 6, 2010
Rousey vs Gomes - March 27, 2011
Rousey vs Tate - March 3, 2012
And Jojed won her title slightly less than 3 years after her MMA debut? Even McMann got her first title shot within her first 3 years.
Granted, Nunes took a MUCH longer road (debuted in 2008), but pretty much anyone with a world class skill set is going to rise a LOT faster than the typical fighter.
It's easily conceivable that there's multiple fighters who are going to rise very quickly to the upper levels of the sport and push aside some of the older fighters well before Rousey retires. And with Rousey demonstrating the amount of money that it's possible to earn it's definitely a possibility that other fighters in Judo, wrestling, BJJ, MT or boxing could make the transition and move quickly up the ranks.
What I'm waiting for is what's going to happen after the 2016 Olympics. There's a ton of top tier amateur athletes that are considering MMA. Kayla Harrison has discussed the possibility of entering MMA after she finishes her next Olympic run and I doubt she's the only one who's considering it.
Legitimate contenders can appear a LOT faster in WMMA.