Now it's understandable that they wouldn't rehash the plot, by making more Zombies or Cat Creatures, save for the mysterious fourth one that's been prowling the Island. However what made the original much more interesting was that it forgo the usual formula of monsters being men/women in masks in favor of showing what it would be like if Mystery Inc. stumbled upon a real supernatural mystery. One that almost got them killed no less, in comparison to the usual formula where even the monsters go out of their way to help them in a running gag. Instead, not only do they rehash the usual formula, by making the Zombies and Cat Creatures the usual masked crooks after gold, not only do they not stick to the continuity of the first movie, where the Gang are all adults whom on top of solving mysteries had their own respective jobs (Save for Shaggy who got fired.), but much of the movie tip toes around whether or not they truly believed what happened to them the first time around actually happened. In essence, they, specifically Fred and Velma whom were the skeptics the first time around dismissed the original events as masked crooks at best and hallucinations at worst. The movie was nothing more than a cash grab designed to cash in on fans of Zombie Island and they didn't even do it right. What would have been a better plot if they insisted on the Scooby Formula would be this: The Gang is invited back to the Island, like before. Only it's now a tourist attraction. They would initially suspect Snakebite responsible for the scares, since he himself hates tourists. So naturally the keep an extra eye on him, same way they suspected Beau in the first movie. Only to find him once again, being a red herring. Then they suspect that somehow Simone, Lena and maybe Jacques have come back from the dead. Which was why the Zombies attacked in the first place, to warn the living away from them. Of course that would also be a red herring because they'd check the secret passage way and find the dust and pendant where the cat creature trio used to be. Then here's where the Scooby formula would kick in. The Zombies and fake cat creatures would turn out to be part of a cult, whom heard the story from both Daphne's News and Beau's testimony. They sought the island in hopes to get the same immortality, that the trio got only for them to not get it, because the secrets died with Simone and Lena. So they lured Mystery Inc, in hopes they had some knowledge at best or at worst they'd sacrifice them to avenge the trio. Just as they were about to do so, the real Zombies show up along with Simone's cats (Whom made peace with Scooby in the first movie.) and subdue them until Mystery Inc can have them tied up.
That would have been a better plot, but nope. Men and Women in Masks and the slight hint of a real cat creature.