Maybe you already thought of these, and I can't see all you intend to do yet since it's a sketch:
the trees on the angelic side are full with leaves as I can see, and the tree on the demonic side is without leaves. Watch out that you don't make the angelic side heavier than the demonic side. Maybe add some demonic fire in the tree to balance it. Her angelic wing is also going to be more full and big ( due to feathers ) after all. I would try to get a balance. Maybe different type of clothes each side can help you find the balance.
If I could get a glimpse of her evilness too from the face on the demonic side, It can be nice for the concept. After all, the face is what get's the most attention usually.
(or just a wild idea, to mix the bad and evil at random. that can also suggest that it's all part of 1. and things aren't always as they seem at first. Like an angel wing with demonic face. etc

)
these are mere quick thoughts and suggestion upon analyzing what I saw. Your the artist... hope this helps. Now go, and make me proud out there. xD