@EllionThat works well for me.
Especially the seer thing. About Shai's divination though, quietness and time would be necessary to more detailed and long term divinations, but for the tarot one (basically functioning as a "sixth sense") would it be ok? Its not terribly overpowered... it can reveal only small parcels of information (contained on someone's post).
About Shai, she is a rather... unique witch. Due to her affinity with the arcane, the reactions she has to lavender, silver & etc would be far worse than normal witches (I'm assuming that these substances interfere with the flow of magic around them.).
Due to the fact that she is basically an unending fountain of mana, she wont get tired by casting multiple magics (bigger magics will still take time to be performed. That is a no brainer. At least one post "preparing" to cast a bigger magic.). Her body is highly resistant to "magic overloads / outburns" (due to not being human at all) but it can still happen, and in her case, when it happens its more severe than normal, often leading to her hands / body being burned by raw mana, forming big, purplish blue wounds and very often rendering her unconscious. She has access to healing magic, but in order to the flow of mana passing through her body be normalized again, it would take some time (while she is still unconscious) and healing these magical wounds can only be done with magical power. If she doesn't receive "first aid" and basic medical care while unconscious after an Arcane Outburn, she will probably die. A normal witch can use magic to heal her, but it would be very consuming to the witch.
Oh, and about her being an Arcane Node, I think that it would be very interesting, making Shai a strategic "possession" to each one of the parties.
Witches would have an unlimited access to Arcane Energy, drawing directly from a Node. That would allow them to cast bigger magics without exhausting themselves.
Hunters would greatly benefit from Shai's ability to make the witches less powerfull.
Well, I'm not saying anything, but if someone captures Shai, using her as an "amulet" to boost nearby witches powers....
Or maybe a hunter who earns her trust and admiration (or perhaps the good and old torture) to make her use her ability...
Well.. Those are only thoughts too :-D
I don't play to Shai to be a "predator" by any means. All of her abilities will be used mostly defensively. The dices and the star fall are the only exclusively offensive abilities she has.
My idea was to her fulfill a strategic role on the rp.
Both witches and hunters want her for her powers. How they will get her is up to them.