I actually am hoping for a bit of depth. If it's a prequel, we might find that Saul started in a different place then where we see him now. If it happens after the series, it might actually be a reverse Walter White story: end up becoming involved in a case where he can use his sleazy tactics to help a good person. Vince Gilligan himself pointed out that as scummy as Saul is, he never tries to disguise who he is, like most of the characters in the show. Maybe this will allow us to either see us lose his values or gain new ones, and in both cases, see his world thrown into chaos because of it.
Also, please bring back Bill Burr and put David Cross in a recurring role.