The villains blowing up the Mystery Machine could be taken as an attempt to kill the gang in and of itself, but then why would they set it to blow up while they were still some distance away? It wasn't an actual trap; it was the bait that would ensure Fred's personal stake in the matter would override Velma's reservations.
For the four bad guy choices. Despite the jail time, a high-profile attorney, a shipping magnate, a 70s rock band member and a tavern entrepreneur would probably be some of the best choices of Scooby villains to have enough money to buy up the castle.
The idea of a Scooby villain social network seems hokey, until you start counting how many potential people there are to join it over the many decades of Mystery Incorporated solving mysteries.
While the Scooby revenge group is specifically to Scooby, earlier in the movie there also is a site that reports on meddling kids. A subtle nod to fact that Scooby's Follow the Leader series are also out there.
As the main page pointed out the opening of this movie uses what appears to be a random clip from Abracadabra Doo with Madelyn Dinkley walking with the gang. But it's there for the viewer to give a hint right from the get-go something is wrong with the mystery when the lawyer says Velma is the youngest of the Dinkley's.
Further the opening credits is set up like a stalker shrine with what appears to be a Dinkley family tree but no Madelyne. It's an in-universe mistake on the bad guy's part that a super Scooby fan is meant to deduce. Pretty brilliant idea for the movie where villains finally return.
You'd think that instead of just telling Velma about the inheritance, the family lawyer would've also told her parents, since they're the authority of the family and he should be reporting directly to them. However, it makes sense as this was all a ploy to trick Mystery Incorporated into visiting the castle.
Several of the train cars fall off the train before the gang gets the passengers to start running towards the front car. We have no idea how many people (if any) were in them.
Daphne's Heroic BSoD over her "curse" could have had additional causes than simple vanity about her appearance. Daphne is a model. Her job is based around looking perfect (by the narrow beauty standards of modern America), and now that's suddenly not an option. She lost not only her looks, but also the career and fanbase she worked hard to build in one fell swoop.
The mere idea that past villains would return to try to take their revenge on the Scooby gang is scary. But what makes it even more terrifying is that the ones we’ve seen in this movie are but a small fraction of the countless criminals that have been caught by the gang. Who’s to say that other villains won’t have the same idea of teaming up to try to take their revenge on the gang in the future? And what if they all decide to come at the gang at the same time? Just imagine, instead of just four of them like in this movie, there could be an entire Legion of Doom of Scooby-Doo villains out there scheming, plotting and patiently biding their time in the shadows, just waiting for the right moment to strike at the Mystery Gang…
