I think that Nesta going to the Illyrian mountains will be the best thing that happens to her, and the girls/women there. She is hard as nails...not to mention that mysterious power that's been hinted at.
She's also got a severe case of PTSD... of guilt (she watched the father she hated for most of her life get killed...right after finding out that he loved them/her so much...that he finally got up and did something for them...and in such a grand way).
Then she watched probably the only man/male she's ever truly cared about almost get killed.
Freya said it best. Nesta feels everything deeply, and I think that is why she has been hit so hard by the events of the series. Then there's Cassian....I do not think there could be a more perfect mate for him than Nesta. And him...her. She's strong, hard, fierce and has proven that she was willing to give up her life protecting him and her sisters.
That strength of hers is what is going to finally push the females in the Illyrian camp to be stronger and more assertive. Nesta will get to work off that edge and work through her demons and fear there also.
Cassian and Nesta to me are different sides of the same coin.They mask their pain with ferocity, anger and both of them are innately strong-willed and powerful.
I cant wait to see what the next instalment brings.