Thinking about how I would write an adult Scooby-Doo series, because I think it can be done.
The first thing I’d do is make the characters actually be adults. Still young, but adults, in the mid to late 20s range. Mystery Inc. is a private detective type business that they run together. In this universe, the supernatural/ghosts/etc are real, but not necessarily common, so when they take on a case, the culprit might be a person disguised as a monster, or it might actually be a real ghost. The stakes can be higher; sometimes a bad guy is legitimately trying to kill them. Sometimes the mystery they’re trying to solve is a murder. Sometimes they actually get hurt on their cases.
Fred: the core of Fred’s character should be that he’s incredibly kind. Like, give a stranger the shirt off his back kind. The “Fred can’t talk to potential clients because he might take a case for free and we need to eat” kind. He’s an honest and good person and sometimes gets himself into trouble because he assumes other people are too. While he’s not very good at reading people or noticing ulterior motives, he’s brilliant when it comes to mechanical or engineering type stuff, so he’s the one who keeps the mystery machine running, builds their gadgets, and of course, designs the traps.
Daphne: she comes from old money, and her parents absolutely despise her life choices, to the point where they haven’t officially disowned her, but they have basically cut her off, so she doesn’t actually have access to any family money. Growing up wealthy has granted her a variety of skills, including speaking multiple languages, horseback riding, and fencing. She’s very into fashion and jewelry (even if she can’t afford it anymore) and has extensive knowledge of both that can occasionally provide a vital clue in a case. And even though her parents have cut her off, Daphne still has a wide network of contacts she can ask for favors sometimes, because she’s personable, and people tend to like her. Daphne is also very emotionally intelligent, and is usually the one who can spot when someone is lying to them.
Side note - I ship Fred and Daphne, so I think I would start them off as an established couple for this universe. Dating, engaged, married, I don’t care. They are stupidly in love, ride or die for each other. There’s no will they, won’t they, no worries about cheating. They are in a healthy, happy, loving relationship, and no one (not even Daphne’s disapproving parents) are going to mess that up for them.
Velma: she is the forensics nerd who sometimes gets super excited about the wrong thing at the wrong time (”He was mummified in seconds? That’s so cool!” “Velma! His wife is standing right there!” “Oh. Sorry.”). She’s not purposely insensitive, she just gets laser focused on her work and forgets to filter herself sometimes. She’s also the one who can get so fixated on solving whatever mystery they’re working on, she’s willing to bend or maybe break laws. Is breaking and entering really so bad? Not if it gets them answers.
Shaggy: he is still the comic relief, but he’s the comic relief by being the only person in the group that actually has common sense. He manages the business’s finances, he’s the only one who knows how to cook, and the others tease him for being a coward sometimes, but Shaggy maintains that if a ghost with an axe is coming for you, running is the only sensible option. He should also have a range of random knowledge that sounds useless, but sometimes saves the day (ex ventriloquism, origami, the history of spoons, etc).
Scooby: as this is a universe where supernatural creatures exist, Scooby is an ancient eldritch type being that took a shine to Shaggy when he was a kid, and took the form of a talking dog to befriend and hang out with him. Aside from the talking dog bit and not aging, he never uses his powers in a way that anyone notices. The audience is not told upfront that Scooby is an ancient eldritch being; it should slowly be hinted at throughout the series so the audience put it together, but the characters never realize it. Scooby genuinely considers Shaggy to be his best friend, and cares about the rest of the gang too.
