Beginning with Scooby-Doo! Mystery Incorporated, the Scooby franchise has had a considerable amount of Easter eggs. Most prominently, classic villains makes cameos quite often in the franchise, particularly a select group of Where Are You villains. It has arguably gotten to the point where Easter eggs have become a regular part of the franchise. It made sense for Mystery Incorporated to do this, as it was sort of a love letter to the franchise that expanded on elements people loved about the original series. It was also quite soap opera-ish and included some mature jokes, which are not things children are typically looking for in television shows. Given this, the series was arguably at least somewhat catered towards the adult fanbase. Be Cool continued this trend a bit by regularly including Easter eggs within episodes, as well as reimagining critically acclaimed episodes featuring redesigned classic villains. Guess Who took a bit of a different approach to the matter by trying to make the tone match with the original Where Are You show almost exactly. The DTV films have also commonly incorporated classic Where Are You villains during the opening credits.
In general within media, I love Easter eggs. They can be such fun bursts of nostalgia and can bring back good memories. Scooby-Doo! Mystery Incorporated sprinkled tons of references throughout each episode, which made it enjoyable to try to catch them all as you watched. I think the surplus of Easter eggs worked well with SDMI's format, since it was a show that took elements of the franchise that fans loved and tweaked them, such as making Velma's parents run a Spook Museum full of monster costumes representing Crystal Cove's supposed "lore" (which were actually just Easter eggs of classic villains). I also loved what Be Cool, Scooby-Doo! did with reimagining classic episodes, giving viewers the fun of searching what parts of the classic episode influenced how certain elements of the reimagined episodes were developed. Personally, I don't enjoy the way Guess Who and some of the DTVs incorporate Easter eggs quite as much. Especially in the first season, the Easter eggs felt constant. Along with the nostalgia that felt a bit forced in my opinion, the use of Easter eggs got to be a bit much. One of my favorite things about Easter eggs is that they are often obscure, or they reference things that the audience hasn't seen in a long time.
Guess Who tended to reference things that had already been referenced quite a bit, for example, having Redbeard, Charlie the Robot, Captain Cutler and The Creeper appear in "A Haunt of a Thousand Voices!" To me, that felt like less of an Easter egg and more like an "ugh, those villains are appearing yet again?" I think it would have been much more fun, at least personally, if they would have included some more obscure villains in the episode that hadn't been used in a long time (take the Wax Phantom for example). I thought SDMI and Be Cool always had a good mix of Easter Eggs, but it felt like Guess Who mainly pulled Easter Eggs from Where Are You with the exception of a few times, which got to feel a bit old after a while.
One of the biggest problem I have with Easter eggs is that it's often old villains making cameos in some way. I am fine with them doing this, but I would prefer they expand their pool of villains. How many times have we seen Captain Cutler, the Miner 49er, Charlie the Robot, the Ghost Clown, the Mummy, the Zombie, Space Kook, Redbeard, the Green Ghosts, and The Creeper, among others, appear now? In my opinion, the references to these particular villains has begun to feel a bit forced. Part of the charm of an Easter egg is not expecting it for it to pop up, and making illusions to the same pool of series and episodes begins to feel expected after it keeps happening.
Bringing it back to the conversation about nostalgia driving people liking certain Scooby-Doo shows, nostalgia in general is huge right now. Nostalgia is a very marketable concept, because it brings back fond memories from a simpler time, whether that be childhood or some other happy point in a person's life. This gets into a much broader concept than just Scooby-Doo, but I would argue sometimes nostalgia can get in the way of people liking updated incarnations of classic shows. With Be Cool, Scooby-Doo!, many people refused to watch the show just because of the designs. The common refrain from a lot of critics of the show was "this ruined my childhood!" I think what these people actually mean to say is "I'm concerned this will change the vision of the show I had in my head from childhood." It's the same refrain with things like Daphne & Velma. That movie is arguably very different and is basically Scooby if it was a Disney Channel Original Movie, so it's understandable some might not like it if they don't like that genre of film. However, nostalgia often drives fear of change, which in turn makes people not give shows a chance at all. On this topic, as much as I hate to say it, I think there is a very small chance Velma will be successful. It's sad, because the show isn't even out yet and we know little about about, but so many people have already written off the show in their heads that I don't think it has much of a chance. The appeal of cartoons for some fans is the nostalgia of the Saturday Morning experience. Most likely, this isn't going to be anything like what they remember the show, so therefore it will probably get a lot of hate. In some cases, people end up just not liking things for different reasons unrelated to nostalgia, but I do think nostalgia is a common reason why adaptions of Scooby that are "too different" are disliked.
Tying these two concepts together, for me, what makes the best nostalgia is references that allow the show to grow. Guess Who and some of the DTVs focus a lot on the same pool of episodes. Particularly Guess Who tried to make the show's tone exactly what it was in Where Are You and The New Scooby-Doo Movies, but I think most people can agree it didn't really capture that tone. What I liked most about the Be Cool and SDMI references is that they used those Easter eggs and callbacks while also doing something very different - Be Cool more directly by reimagining classic episodes, and SDMI through working the Easter eggs into the lore of Crystal Cove. I think the best references and Easter eggs are the ones that genuinely surprise you, and don't try to shove the nostalgia in your face, or pull from the same pool of shows/villains over and over. I am not saying that every reference in Guess Who was terrible and you should never watch it - but in a general sense, I feel the references were incorporated better in the previous two shows (bonus points if you got the pun there). I think SDMI kickstarted future shows and DTVs having frequent references, but with the exception of Be Cool, the references weren't always as cleverly worked into the lore of the show in a pleasantly surprising way. To be clear though, I'm not saying I hate Guess Who. There were a lot of things about the show I enjoyed; I just liked Be Cool and SDMI quite a bit more.
Personally, I most love references to things you never thought you'd hear about again, or haven't heard about in forever. One recent non-Scooby example is The Gruesomes' appearance in Jellystone!, which I recently praised in a fun fact post. For those not familiar with The Flintstones, the Gruesomes are weird, Addams Family like characters who briefly moved next door to Fred and Wilma for three nonconsecutive episodes. I honestly never thought I'd hear about them again since they hadn't appeared since the 1970s, so it was such a pleasant surprise to have them randomly pop up again. They are worked into the episode by appearing in a horror film Magilla Gorilla is going to see, which is what sets up the episode's plot of a burglar breaking into his store. I feel like this is a perfect example of how to make a good reference, because it's completely unexpected, and also worked excellently into the plot.
The question of "how much nostalgia is too much nostalgia?" is ultimately something I think each person has to answer for themselves. I think it makes for a much richer show when nostalgia is somewhat limited, because it makes it feel more special. I really like when the franchise tries completely new things (Scooby-Doo! Mystery Incorporated) or puts a new spin on certain elements of the franchise (Be Cool, Scooby-Doo!). That's what keeps it interesting and allows room for change and growth. This is one thing I dislike about Guess Who: it tries to make the show too much like the "classic" years, and doesn't do anything different or particularly unique. That exploration of different elements is what keeps things interesting, in my opinion; I wouldn't just want to have a scenario where we're all like "alright, guess it's time for season 49 of Where Are You!" A lot of these "off" Scooby shows and films, like Daphne & Velma and Get A Clue are nothing like Scooby, but I enjoy the experimental nature of them and they're fun for what they are. Of course, there are some instances where it just doesn't work very well, due to poor writing or a bad concept, Arabian Nights or the Scrappy shorts being good examples. In general, it's fun to see different elements become more prominent in different series.
To summarize this article in simple terms, nostalgia is good in moderation. However, if it is used too heavily, it can hold series back or make them feel boring or bland. I would prefer new series to bring something different to the table that hasn't been explored in the franchise previously. What's New, Scooby-Doo? did it by providing a modernized 21st century update to the characters and atmosphere of the franchise. SDMI did it by adding a more mature tone, relationship drama, an overarching plot, and reimagining certain elements of the original show in a different timeline. Be Cool did it by focusing more on character dynamics and providing a more humorous spin on the show. There is always the risk of experimental concepts not working, but I would argue never exploring anything new within a franchise also comes with a cost; that being the plot becoming stale. When series and films retread old ground, and use the same types of references over and over that the nostalgia begins to feel too much. As long as references are done in decent moderation and the series/film is exploring something unique, I think nostalgia can be a very positive thing. At least in my opinion, a healthy balance is very important when incorporating nostalgia into series.