I have an idea for a focus, but I wanted to ask about it first before going ahead with it.  I'm thinking of making it a little tattoo on Cassandra's offhand, probably just a dragon wing or horn of some sort covered in blackbriar thorns.  For now she'll just grab a small orb or shard of crystal to serve as a temporary focus, but she'd want a more personal touch, and a tattoo because why the hell not lol.  While it'd make it considerably more difficult to completely suppress her ability to cast spells with such a mark, it could easily make her much easier to track down if someone of sufficient magical skill scry her through the tattoo, cut off her hand, or what have you, to make it a bit more 'balanced' given the obvious advantage there.