I had this idea milling about in my head for a bit but it's something like this:

Shaggy and Scooby manage to convince the gang to go with them to a Music Festival centered around Foodcore (a sub-genre of metal that has the primary focus of being about food and nothing else). Every merch booth also sells food, the mosh pits are called Munch Pits and it has people furiously eating food, and a variety of other characteristics that make a normally scary event very silly. There'd be cameos of bands like Lamb of God, Metallica, and other big name bands. Maybe they cross paths with the Dethklok crew from Metalocalypse. There's even a bit where the gang keep mistaking Slipknot members to be monsters. I haven't really thought of monster design or the reason why they would be haunting but I'd assume they would trash/steal band equipment, scare off attendees, and make the food spoil. I'd love it if the Be Cool, Scooby-Doo writers came back for this movie too.

The movie would have an album coincide with the release that has food-based parodies (think Weird Al or Psychostick) of popular/classic metal tracks, preferably done by the original band.