Meh, I don't feel like speculating on novels that don't exist right now. How about, instead, I just give you a list of secondary characters I really liked: (spoiler-ish ones in white)
Ship Breaker: Pima and Tool
Who Fears Death: Luyu
The Magicians: Penny. He was the only interesting character in the entire book.
The Hunger Games: One of the sad but awesome things about The Hunger Games is that the first book introduces so many characters, but due to the circumstances of the series, a lot of their personalities and back-stories remain a mystery. Thresh? Foxface? Even Cinna we don't know much about. (Although I think it would ruin it if we saw Cinna's backstory.) Also, I love both Haymitch and Joanna Mason.
Harry Potter: Once again, lots of them, but Ron is hilarious and Hermione is under-appreciated, considering that she saves the day EVERY SINGLE TIME. Also, like everyone, I would love to read a book starring the Marauders.
The Kingkiller Chronicles: Devi. These books need less fairy sex and more Devi.
Neverwhere: Marquis de Carabas