If more books were released, I'd buy them in a heartbeat, even if only to read the margin notes between Harry, Bob and Will.
I don't think we really need any more books, though. One of my favourite things about the books as they are is how much Will says the write-ups are subject to speculation, so groups can add, remove or change what they want. Also, Small Favour ends with the perfect balance of power to start your own campaign, in terms of both PC and antagonist options. Adding more books as the series goes on might risk creating too much of a Dresdenverse metaplot.
Hmmm...That's what I was thinking. But maybe its agenda is to break the laws of magic and (immediate) repayment involves taking consequences (maybe specifically mental consequences--the kind that make you question what kind of person you are).
Just on top of my head I would rate the ability to ignore lawbreaker cost maybe 2. But restrict to Items of Power only. And make the power extremely rare.
Maybe sponsored magic with element of shadow/entropy or something similar. Demonic Co-Pilot is worth consideration.