Agatha's locket isn't stolen. Maybe Moloch manages to stop Omar, maybe they picked a different alley to sit in, maybe Agatha manages to run, whatever. Lilith and Adam are able to pick up and move to a small town as planned when Klaus takes over Beetleburg. Of course Agatha still smells good, so every jager that wanders through flirts with her, and she eventually ends up dating one of them. I am really terribly sad that jagers probably don't care much about the ethics of relationships in the workplace, because I really want to see a jager approaching the generals to ask about the policy on accidentally dating the Heterodyne.
Or at least, Tarvek's plan for dealing with Lucrezia. It works, more or less, and he escapes Sturmhalten with Lucrezia-in-Agatha. He then has to spend the next several months attempting to limit the damage she does, learn about her work, and figure out how to free Agatha without getting caught. I'm sure that will be so very much fun for him.
Agatha is Informed
Beetle and/or Adam and Lilith actually tell Agatha things as she grows up, so she grows up believing Klaus is an agent of the Other, determined to fight him, and aware enough to hide from him. I honestly am not sure how this could end up going anywhere good, but I still want to see what happens, and it seems like such a plausible AU.
Zeetha Finds Klaus
Zeetha finds Klaus early on in her travels around Europa. This could go two different ways, because she could equally easily stick around Klaus and act as his mobile conscience to hit him upside the head any time he starts going too far, or skip merrily off to Paris to ambush Gil. You have a sister, you're a prince, and it's time to begin your Skifandran prince training, little brother! "Skifandran prince training" could take the form of chasing Gil with sticks like training Agatha, or putting Gil in fancy clothes and trying to make him practice being elegant and gracious. Either way I'm sure Tarvek will be delighted to witness it.
And then Lars sticks around while the rest of the circus goes to England, and probably proceeds to not have much impact on the plot, but may ruin Gil's and Tarvek's chances at a relationship, due to Agatha actually having a possible boyfriend who hadn't kidnapped her, threatened her, ordered her to marry him, or gone over her head in an attempt to protect her, and who can describe why he likes her without including "the power of the Heterodyne name."
Tarvek's Great Idea
Child!Tarvek is determined to protect Gil on Castle Wulfenbach, but even keeping Gil as his best employee isn't perfectly safe. So what would be better? Why, Tarvek will just arrange for Gil to marry his sister! Gil's sure to be a strong spark so that will be acceptable, and then he'll be royalty, or as good as! Of course, first that means Tarvek needs to teach Gil to act like royalty, manners, fashion and all....
Gil never finds the family records vault, or maybe they can't get in, or maybe they just don't get caught. However it happens, Tarvek isn't sent home from Castle Wulfenbach, until he's called home years later because his older sister is dying. Of course, by then he's grown up with Von Pinn and the Baron's morals as models, and no time to adjust to thinking of the wasps and killing girls to summon Lucrezia as normal things, so once he figures out what happened to his sister he's horrified. But that still leaves the question of what to do about it....
I can see this going a lot of different ways, and mostly I'm really fascinated by the idea of a Tarvek who still thinks the Baron is an usurper, still wants him taken out, but also is absolutely horrified by what his family and especially his father has been doing, and has to negotiate between those. Maybe he'll try being sneaky and taking over Sturmhalten or the conspiracy, maybe he'll go to the Baron, maybe he'll just ask Gil for help, maybe he'll ship Anevka off to the Great Hospital to save her and there will be fallout from that, I don't know but I'd really like to see.