What do you want to see in a potential Scooby Doo machine?
CASE - There need to be modes themed after different cases. Once started, all of the major shots have a CLUE insert. Some will be lit. Collect enough clues to solve the mystery. UNMASK the villain for a jackpot. I think it makes the most sense to have all of the villains in a single location like the Mystery Mansion. That would allow you to have better toys (trapdoor, secret passage, falling chandelier, etc.). The final wizard mode could be Escape the Mansion. Start the wizard mode by (at least) starting all of the cases, but have it be easier to complete if you’ve unmasked some of the villains.
CHASE - Many of my memories from Scooby Doo are The Gang, especially Shaggy and Scooby, running from the villains. I would especially like to see the classic "hallway chase" where our characters are running across a hallway through a series of doors. If it's a flowing machine, that might be a fun way to display combos. Maybe have a dead end shot also lit for combo. Ending a combo with that shot will be worth bonus points and display our characters hiding from one of our villains.
SCOOBY SNACKS - I enjoy machines that have a simple collection mechanism: fish on Fish Tales, diamonds on Congo, etc. I think the Scooby Snacks would work well for that. I would have a Scooby Snack insert on most of the main shots. Collect so many Scooby Snacks to light a timed shot for extra points...something like that. Perhaps also have a Scooby Snacks multiball.
MYSTERY MACHINE - I would probably use the Mystery Machine as the ball lock. While I prefer less balls in play, it might make sense to have the potential of a five ball mutiball, each ball representing each member of The Gang: Fred (Blue), Daphne (Purple), Velma (Orange), Shaggy (Green), and Scooby. I also like how each of the characters has a color associated with them. This should be incorporated into the inserts.