Nunes has the potential to put up a good fight.
She will need to work on 3 major things;
1) Lack of footwork. She tends to get very flat footed, which does her justice in the power area but will make her an easy target for a Ronda toss or trip after the clinch.
2) The clinch. She is too comfortable working the clinch which as everyone should know by now is Ronda's bread and butter. If she can keep Ronda out of her comfort zone and circle away she might do okay.
3) Gas tank still unproven. Even if she has picked up better Cardio recently, a fight with Ronda will involve lots of movement and breaking off of the clinch. She will have to push Ronda away, and spend a great deal of time being hunted to avoid the clinch. Her striking and her low hands will work against her too because Nunes tends to get a bit lazy and drops her hands a lot which will let Ronda get inside very easy.
Yes Nunes could be a contender, but she has to put in some serious work to deal with what Ronda brings to the table; high pressure clinch work, mobility/footwork, and proven cardio to deal with the the very aggressive and relentless Ronda.