Ronda will likely retire from combat sports within 5 years. She simply has too much going for her to overstay her MMA career. She wants to retire undefeated.
She's essentially cleaned out the 135 division with Holms being the only potential fighter who could provide any competition and she needs 2-3 more good fights before she's ready to challenge for the title.
Justino can challenge for the 135 title as soon as she can make weight, but at 135, the cut will either take too much out of her, or she will have to drop muscle in the months prior to cutting down to 135 for weigh ins.
I don't see the UFC adding a 145 woman's division, so there is a real possibility that Silva will simply run out of matches for Ronda without resorting to rematches of previous squash matches.
That said, I'm sure there are talented teenage girls training in MMA with the intention of going pro, inspired by Ronda to be the future of women's MMA, one of which could be the next prodigy to become the 135 champion. Personally, I'd look to women competing in Olympic level combat sports for that potential future champion.
Ronda has said herself that she thinks one of her greatest advantages is that women don't know just how tough she is until they get in the Octagon with her. She said in an interview that the person who has a good chance against her is Miesha Tate due to the fact that they have fought 3 times now already. Tate could be next or it may be Cyborg but it doesn't quite sound like Cyborg is going after that fight, instead taking the "I'll wait until Dana says something" approach.