One aspect that makes it sad is the backstories of the zombies. They were once normal people with their own lives and loves, but now they are trapped in this undead state. It's a tragic fate for them.
Through the atmosphere. The dark and spooky setting of the island, with its old mansions and foggy landscapes, sets a melancholic mood that implies sadness.
Scooby-Doo was indeed a comic first. The comic laid the foundation for its later success in TV shows and movies. It introduced the beloved characters and their adventures.
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.
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.
One idea could be that the gang discovers the zombies are actually victims of a mad scientist's experiment gone wrong. They have to find a way to reverse the process.