I didn't go too much into the time travel thing because it could be done any number of ways. My thought is more along the lines of, "A villain gets a way to travel in time, he goes into the past to cause trouble. Harry has to follow him, drag him back to the present. Along the way, various things happen that cause little details we've noticed in previous books."
It's not lazy at all, done like that- it's just a time travel story. The inconsistencies aren't the POINT of the story, just a by-product of them.
Obviously it's not going to be "Harry gets supreme temporal power, he decides to go hang out in Proven Guilty."
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Anyway, it doesn't matter. My main point was that the other theories for this thread would make for a terrible story. @namkcas, you said:
Things come back 10 years later in this series.
Which... kind of misses my point. References to old things aren't bad. But a reference, by itself, isn't interesting.
I guess I'm presupposing two things:
1. The inconsistencies in Proven Guilty are numerous, and blatant, enough to have been done as a setup.
2. A setup needs a PAYOFF. Jim wouldn't put that much effort into something if it wasn't going to be part of a good story.
Do you see? There is no (non-time travel) way to make "10 years ago, Ace hit Harry's car" an interesting payoff. It would cover ground we've already seen in Cold Days. We already know Ace was still around, we already know he's kind of ineffectual. He took his swing at Harry in CD, and missed.
Same holds true for the popular Mab theory. We already learned, from Skin Game, that the fae can enter a home if they mean no harm. That information is stagnant, unsurprising. So, if in a book or two, Mab says, "Oh btw, I fixed your model for you 10 years ago. Thought you could use the help, nbd" it would make SENSE, but it would be completely and utterly BORING. It wouldn't make for a good story.
The reason I mention time travel is because it is one of the few ways to make the mysteries of PG interesting, give them some immediacy. After all, there isn't much at stake otherwise, is there? Molly was rescued, the fetches defeated, everyone moved on with their lives.
It doesn't HAVE to be time travel, but it does have to be interesting.