The common thread of the series is that these are the books that Elephant and Piggie like to read. Elephant and Piggie are retired, so this is what they do in their spare time. What will they end up wanting to read? Time will tell. We've got to let that evolution happen as the series goes on. Any return of a character would have to be organic, would have to be, for example, Laurie Keller saying, hey, I really want to do this, and me feeling that there's a story there, rather than just saying, yeah, we can get three books out of these characters.