Annette, I just finished 3.5 and I have to say that I love Lyre, so I sincerely hope you will take good care of him. Dare I say that I would not be opposed to a whole series arch about him when you're done with Piper. This is a great series and I am so caught up in the characters that I don't want to leave them. So, are you planning to explore more of the other characters' stories?
Spin-offs focusing on other characters are certainly a possibility, but won't be my next project after the Steel & Stone series is complete.