http://www.examiner.com/article/once-upon-a-time-rebecca-mader-talks-about-zelena-s-redemption I have a hard time imagining Zelena being redeemed. It could be just the rather bitter frame of mind I'm in regarding my OTP, but based on what I've seen (which, admittedly, I have enjoyed, apart from the fact that Rumbelle is being shunted to the background to make way for a kissing curse) she doesn't seem to me to have the appropriate character. Before going dark, Regina and Rumple actually loved -- and in some ways it was that love that drove them to do what they did -- and still loved after they had crossed the line. Rumple was always searching for his son, Regina did still mourn her father (even his death came at her own hand) as seen in that she named Henry for him. She also loves Henry, and as her arc progressed she learned to love him properly as opposed to possessively.
Zelena has none of that. I have seen no evidence that she has ever loved or truly cared for anyone beyond herself before going green or presently; there is no crack in her armour of envy, all there is is herself. Even if she succeeds I doubt she'll be happy. Envy cannot be satisfied because all it wants is to possess.
As a writer, I would also like to see her remain a villain...we have two compelling story arcs which are shamelessly manhandled and to throw a third into the mix would dilute it too much, I think. Also, not everyone can be saved. It's a show based around fairy tales and while fairy tales do have stories of redemption, there is also the flip side of the coin where they do point out not everyone can be saved.
Anyhow, those were some thoughts...