One idea could be that the gang discovers a mystery related to an ancient curse in their hometown. Maybe there are strange happenings around an old, abandoned mansion that was once the site of a dark ritual. Another could be a mystery involving a new, mysterious character who seems to know more about the gang's past adventures than they do themselves.
One idea could be that the Kiss band members team up with Scooby and the gang to solve a mystery at a spooky concert venue. The strange happenings could be related to a cursed instrument that causes supernatural events.
One idea could be that Naruto and the gang from Scooby Doo team up to solve a mystery in a ninja - haunted village. Naruto uses his ninja skills like shadow clones to search the area while Scooby and the gang use their detective skills to look for clues.
A cool concept could be a possession story. A powerful supernatural entity tries to possess members of both groups. Scooby Doo and the Mystery Skulls have to find a way to stop it before it's too late. This would bring out different reactions from the characters. Scooby and Shaggy might be more panicked, while the Mystery Skulls, with their prior experience, would be more strategic in their approach.
Well, perhaps there could be a crossover where Scooby and Shaggy accidentally eat some magic cupcakes in Equestria that give them the ability to talk to the ponies in a special way. Then they discover that there is a mystery surrounding a disappearing magical element in Equestria. The rest of the Mystery Inc. gang shows up to help, and they all join forces with the ponies like Rainbow Dash and Fluttershy to find out what's going on and save the day.
One idea could be that the Winchester brothers are hunting a ghost in a town where the Mystery Machine gang also happens to be. The Scooby gang initially mistakes the Winchesters for bad guys but then realizes they're all after the same spooky entity. They could team up, with Sam using his research skills and Dean showing the Scooby gang how to handle real weapons (in a non - dangerous way, of course).
A really cool concept would be a fusion of the two worlds. For example, the villains from the Naruto world could be using Scooby Doo - like disguises to deceive people in the Hidden Leaf. It could be Orochimaru in a mummy costume or something. Naruto and his friends would have to use their ninja skills and also learn from the Mystery Inc. how to unmask the villains by looking for clues.
I'm not sure specifically about 'lemon' fanfiction (which often implies inappropriate content). However, there are many great Scooby Doo Mystery Incorporated fanfictions out there that focus on the mystery - solving adventures, character relationships in a more family - friendly way. You can check out popular fanfiction websites like FanFiction.net or Archive of Our Own and search for Scooby Doo Mystery Incorporated to find some interesting reads.
In my opinion, Mystery Inc. as a whole is the best to focus on. You can write about their teamwork and how they complement each other. For instance, Velma's brains, Fred's leadership, Daphne's courage, and Scooby and Shaggy's comic relief all work together to solve the mysteries. You could also create a story where they face a challenge that tests their friendship and teamwork, like when they are framed for a crime they didn't commit and have to clear their names while still solving the mystery at hand.
Well, you could write a Scooby Doo fan fiction where the gang time - travels. Maybe they end up in the Old West and have to solve a mystery involving a ghostly cowboy. Or perhaps they go to the future and deal with a high - tech haunted house. You could also do a story where Scooby discovers he has some special powers that help them solve the mystery.
One idea could be that the OC has a special power to communicate with the spirits on Spooky Island in a way that the gang can't. So, the OC helps the gang understand the real motives behind the spooky happenings. For example, the OC discovers that the 'evil' spirits are actually being controlled by a hidden villain who wants to take over the island.