Hey there! Didn't find the time to actually get going with it. I might send you some concepts to choose from tomorrow.
But it might be of good use, if you gave me a little more info about what you have in mind.
That you are aiming for a green ramp-planeswalker is obvious, but do you have any certain ideas of the design you want?
Like, Nylea before her ascension, in some avatar-form, or after (what could possibly occur in the official storyline) the whole god-thingy on Theros collapsed?
Also, "sitting on a rock in a forest with some trees and animals around her" is not very precise. You want a vigilant pose, or more relaxed? With or without bow?
If she's making a fat wave of creatures with one of her abilities it could make sense to actually depict her with at least one of them...