It's a near certainty that New Frontier
is finished as a series, but there's no reason why the characters can't be used again and mixed in with existing canon characters. I wouldn't mind seeing Shelby and Calhoun again, and I absolutely love Kat Mueller's character. Why not have them pop up in upcoming novels? I don't know that any one of them needs to be featured, but there's nothing wrong with having them make a cameo appearance so fans know what's happened to them.