One choice that most impacted the series...
Not getting Susan away from Bianca in time. That's basically it. All Harry ever had to do is not show Susan the invitation and not leave them out where she could get to them.
War averted, let's go back to the simmering cold war until Red Court is ready to try and wipe White council off the Earth
My personal WAG is that the specific decision is to not misuse Amoracchius during the graveyard scene with Lea. She never gets it, and doesn't trade it to Bianca for the Athame. Couple of options can happen from then on. Either:
1. Harry gets taken by Lea for dog duty. He's not around, and Susan is found out and turned during the party. Thomas and Justine may or may not be killed. The Nightmare isn't stopped, or is finally hunted down by Michael, instead. The war doesn't get kicked off too early, and the Reds gear up for a decapitating strike, which happens years later. Lea may or may not still get the Athame; she may trade something else for it (if she even goes to the party), or simply can't accept it without an equal trade (considering that it took Amoracchius to make it a square deal, I think odds are good she simply doesn't have anything big enough to trade for it). She's never Infected (at least not then), nor is Maeve. Aurora is still Infected, though.
Harry's either turned back into a human by Lea after the danger has passed, or Harry figures out a new deal to get back out of it (and, of course, somehow communicates his terms to Lea). I do believe that Lea didn't intend to keep him as a hound permanently, anyway, and take her at her word when she said it would have been for his own good. So I assume that Harry would turn back sometime before Summer Knight. Except this time, the news about Susan isn't that she's alive and half-turned, it's that she's dead or fully turned. This means a couple of things:
A. Susan and Harry never conceive Maggie.
B. Harry's depression is potentially much worse around Summer Knight.
C. Susan cannot be sacrificed at Chicken Pizza to kill the Red Court, so they remain a significant player regardless of anything else that happens.
Similarly, there's no Red Court side plot activity from Death Masks onward, not until around Death Masks. At that point, the Reds attack, and it's devastating. The Darkhallow may or may not have been pulled off (I lean toward not). But the attack on the Wardens happens, except this time they're totally unprepared for it. Rather than losing half or three-quarters of the Wardens, the Reds manage to take down just about all of them. At this point, Peabody's manipulations are employed to even greater effect, and he's able to leak the location of the Senior Council and organize an effective assault. SC members are killed, maybe all of them.
It's also worth noting that, without the Red Court assault on Archangel, Eb isn't on the Senior Council at this point, because Simon's still there. So Ebenezer winds up on the Senior Council then, along with wizards like Klaus the Toymaker, simply because they're the most senior wizards left.
The war goes really badly.
2. He manages to weasel out of the deal some other way. Michael has Amoracchius handy during Bianca's ball. It changes the outcome of events so thoroughly that Dresden, Susan, Thomas, and Justine all make it out alive! Everything seems jim freaking dandy.
Except the Nightmare remains active, and Bianca's gone full cuckoo bird. She manages to get Susan anyway, through the Nightmare. Rather than turn her, Susan is simply killed, days or even weeks or months later. Harry is unable to pin the murder on Bianca, but attempts to resolve things through the Accords anyway. It fails. Harry becomes totally disenchanted (heh) with the supernatural world as a result, and takes a darker turn. He murders Bianca for revenge. He starts picking off Reds any chance he gets.
The White Council catches wind of it, and starts trying to nail him for breaking the Accords. Harry starts rules lawyering them. He manages (cuz, again, he's much less scrupulous than current Harry) to start making real money, and just starts paying weregilds for each dead vampire (or something similar).
But it becomes an open secret that Harry is fighting a guerrilla war against the Reds. So he becomes a major target, just like normal, except now the White Council hangs him out to dry much more thoroughly than they've tried to in the past. Morgan in particular goes out of his way to make Harry's life as hard as possible. He's not branded a warlock because he's technically not breaking the Laws of Magic, but he's instigating conflicts with the Reds.
Harry hates the Red Court so much that he starts taking advice from Mavra, who never got into a confrontation with him at Bianca's party since Michael was Armed. Mavra's playing her own power game, trying to screw with the Reds (and possibly spread Infection amongst them). In exchange for her help, Harry gets her the Word of Kemmler much earlier, and they manage to avoid the Darkhallow conflict completely. So the Heirs of Kemmler are still running around. In fact, Marcone might have found out about the Word from Bony Tony, and recognized the danger it presented. So the conflict around the Word may have been between Marcone and Harry/Mavra.
Meanwhile, the Reds plan their attack on the White Council, and it's as devastating as I hypothesize above. Part of Harry's problems are solved, because what's left of the White Council doesn't have time to worry about him. Plus, you know, he's been killing Reds for a while anyway, so they kinda look the other way while he continues his thing. But Harry's made so many enemies by this point that it doesn't ultimately make a difference, and he's hunted by various power players (mostly working for Nemesis). That leads Harry to where he is at the start of Mirror, Mirror.
Various changes to the timeline I predict:
1. Molly is either dead, or is not the Winter Lady. I assume she was killed by the White Council.
2. Murphy and Harry never learned to really trust one another. The Summer Knight Wal-Mart conversation never happened. In fact, Harry might well be a wanted criminal.
3. Marcone was left to defend his territory from the Reds. What he saw during the ongoing conflict changed him. He probably didn't transition to legitimate businesses, but I can totally picture him mobilizing his resources to fight monsters, since nobody else is doing it. We kinda see that in Even Hand and the Chicago Alliance meeting anyway, except Mirror Marcone isn't playing power games, he's operating a true, genuine, and honest protection racket.
4. Thomas was killed, and Mirror Harry may never have found out about their relationship. He either dies during Grave Peril, or Blood Rites (BR death is a possibility because Murphy may not have been helping Harry, if the events of that book even happened in that universe). Justine's fate probably matches Thomas's—but either way, I don't see them having a baby.
5. Morgan is still alive, almost without a doubt. He's running his own guerrilla war against the Reds with whatever resources he's got, and he's better at it than Harry. Still hates Harry, though.
6. Eb is probably alive. He might be leading what's left of the White Council if the Merlin bit the dust. Regardless (and this may or may not matter), Peabody didn't have eight years to dose him with mind controlling ink. So Eb's mind may have never been screwed with.
7. Michael might have been killed during Death Masks.
8. Butters is dead if the fight for the Word happens.
9. The Alphas are still around, but a bit different, depending on what Harry does with them. It's possible they're more aggressive in their vigilante activity, and it's also possible they're not allied with Harry at all.
10. Harry never gets Mouse.
In fact, there might be more evil Mouses (Mice?) running around, like in Zoo Day. Mouse might be one of them.
11. Maeve may or may not be Infected, it's up in the air. Depends on what happened with Lea, and whether or not
she gets Infected. Regardless, she's still alive, I think. If she's dead, then Sarissa is the Winter Lady.
12. Lots of others I can speculate about, but you get the idea.
So that's a pretty big WAG. In fact, it's like thirty separate big WAGs rolled into one. But I think that the crux of the Mirrorverse's differences boils down to: "This is what the world would be like if Harry's primary motivation for over a decade was fueled entirely by hatred and revenge instead of sadness and regret." Because, personally, I can see a revenge-focused Dresden losing his desire to defend those who can't defend themselves. We see bits of it come out a few times, like the ghoul attack at Camp Kaboom, or burning Bianca's mansion in Grave Peril. If he was in that state too much, well... things could be a lot different.
