In Scooby-Doo & Guess Who? , two episodes were written as direct sequels to the original episodes in The New Scooby-Doo Movies. Previously, in the eyes of many fans, WB's track record at creating sequels had been very poor. Scooby-Doo! Return to Zombie Island and Scooby-Doo and the Curse of the 13th Ghost created numerous continuity errors, and made all the supernatural elements of each series fake, despite that they were real in both of the original shows. Moreover, Velma even went as far as to argue everything that happened in the original films was fake, which many fans found to be unnecessary.
This caused me to be very skeptical about the Sandy Duncan and Cher episodes when we first heard about them, because I was worried that it was just going to be another repeat of the mistakes of Return to Zombie Island and Curse of the 13th Ghost. However, I was very pleasantly surprised. I felt both films were perfect sequels to their respective original episodes, and it honestly made me wish that the two 2019 sequel films were written in a similar way.
So, what was it about these two episodes that nailed the "sequel" aspect so well? I would argue that its the following two components:
1. Staying true to the source material: Both the Sandy Duncan and Cher episodes directly referenced the source material all throughout the episode, without creating any unnecessary continuity errors. While Curse of the 13th Ghost and Return to Zombie Island took aspects from The 13 Ghosts of Scooby-Doo and Zombie Island, respectively, both films picked and chose what they wanted to follow and discounted the rest. While the basic premise of both films was sort of used, a lot of creative liberties were taken, such as Curse of the 13th Ghost saying that Daphne, Shaggy and Scooby's adventure was "over a summer." Scrappy was also completely ignored, and even made into a joke ("What's a Scrappy?"). For a good sequel, I would argue that you cannot simply pick and choose what you want, while pretending the rest didn't happen. "The Dreaded Remake of Jekyll and Hyde!" used the gang's previous adventures with Sandy in "Sandy Duncan's Jekyll and Hydes" as a foundation for the episode, as did "Cher, Scooby and the Sargasso Sea!" to an extent. Nothing was discounted, ignored, or altered for the episode; rather, they drew upon all the elements of the episodes to make the sequels stay true to the original. I suppose you could argue Sonny not being mentioned in the Cher episode is an omission on the writers' part; however, I think it would have been kind of sad for them to have to acknowledge Sonny's tragic passing (for those who don't know, he died in a horrific skiing accident at the age of 62, in 1998), so I understand why they didn't. The difference between not acknowledging Sonny in "Cher, Scooby and the Sargasso Sea!" and not acknowledging Scrappy in Curse of the 13th Ghost is that Sonny actually had a good reason not to be mentioned, whereas Scrappy's lack of acknowledgement was clearly meant as a punitive joke, which there was no good reason to make.
2. Sticking to the tone Perhaps my biggest problem with Curse of the 13th Ghost and Return to Zombie Island was the drastic difference in tone. The 13 Ghosts of Scooby-Doo had a dark, creepy atmosphere to it, even if it did get a little silly sometimes, and Zombie Island is beloved throughout the Scooby fandom due to its dark and mature tone. It seems the writers acknowledged none of that for the sequels, however, and just made both sequels like it was just another DTV from the current era. To make a truly good sequel, I would argue that the tone of the original needs to be taken into account, which it didn't feel like they did at all for the two 2019 sequels. It would be like a writer coming along and saying "you know how amazing people think Star Wars is? Well, screw the tone of the original, let's make it a soap opera!" In the case of Return to Zombie Island, Moonscar Island looked completely different from what the original was. With Curse of the 13th Ghost, Vincent's home was randomly declared to just be a "Air Boo and Boo" that he happened to be staying at. To use the Star Wars metaphor again, that would be like saying the Millennium Falcon was just some random spaceship they rented from a guy or something. Vincent's castle was a pretty core part of The 13 Ghosts of Scooby-Doo and appeared in a lot of episodes, so for them to just ignore it completelIt feels like the writers wanted to advertise the films with the flair of a sequel, without actually putting much work to make sure that the elements of the original were there. Especially with Return to Zombie Island, pretty much nothing is kept the same with the exception of there being zombies and cat creatures on the island, though there's not much lore as to why they're there. With the two Guess Who sequels, it felt like they really dove deep into the original episodes to ensure that the tone and the setting was the same. The Sandy Duncan sequel felt very true to what the writers of the original episode would have done if they would have made a sequel themselves, which is how I would argue a Scooby-Doo sequel should be made. The episode took place on the original set the gang was on, incorporated the element of there being numerous monsters on the set, and even framed the film they were making as being a "remake" of the original. I also really enjoyed how they made the sequel's Mr. Hyde be a combination between the one from the original episode and another classic Hyde from the Scooby-Doo universe (the one from "Nowhere to Hyde"). The Cher sequel did the same. It took place on a boat, included shark men as the villains, and took care in trying to match the tone of "The Secret of Shark Island." This is why I feel the Guess Who sequel episodes were miles better than the two sequel films of 2019. Both of them attempted to match the tone of the original episode, and incorporated various elements that were used by the original episode's writers throughout the episode, rather than just having a vague resemblance of the source material like the two sequel films did.
I know many Scooby fans are a bit skeptical about sequels to anything now after Curse of the 13th Ghost and Return to Zombie Island, I would honestly be open to more sequels if they were done as well as "The Dreaded Remake of Jekyll and Hyde!" and "Cher, Scooby and the Sargasso Sea!". I thought both of those were incredibly well-written and felt super true to the original episodes. I hope if more sequels are made someday, either a movie or as part of a series, that the writers take inspiration from the what the Guess Who writers did with the Sandy Duncan and Cher episodes.