Minor characters developing into more is one of the biggest reasons I like long series in general and Dresden Files specifically. Just off the top of my head:
SF: Murphy, Morgan, Susan (and a whole bunch more because first book...)
FM: Alphas
GP: Thomas, Michael, Charity
SK: Mab/Eb/LtW/Elaine
DM:Sanya/Molly/Ivy/Nic
BR: Lara
DB: Butters/Ramirez/Luccio
PG: Madrigal (Kind of a stretch, but I am drawing a blank)
WK: ?
SF: Tessa/Eldest Gruff
TC: Private Eye (I know he has only appeared in one so far, but I think he will come back)
Changes: Vadderung
GS: Evil Bob (did he survive Omaha Beach?) and arguably Bonnie
CD: Sarissa/Titania
SG: Hanna (I know she might be dead, but since this is the latest book who knows)
Once again, that is just off the top of my head. I'm sure others can come up with more.