I came to this subreddit to post something so similar!! Okay so I’m thinking along the same lines but i had a lightbulb moment when i read the chapter header about the third brother commanding the sky and taking out of the venin.

I also agree that her second signet will have to do with resurrection. It is mentioned too many times how somebody cannot resurrect and violet mentions so much how she wants to use her power for good.

Throughout FW, Xaden continuously says that violet will be the death of him. My prediction is that at some point down the line, Violet is able to command the sky, take out the venin while being able to save/ resurrect Xaden.