Scooby Doo and his human co-horts riding the Mystery Machine are long-time residents of the Tooniverse. And they've been integral in linking many other animated series and bringing real-life celebrities to animated life.
The Scooby Gang also now exists in the movie universe as well, thanks to two movies starring Sarah Michelle Geller, Freddie Prinze, Linda Cardellini, and Matthew Lillard.
Those actors won't be involved in the next installment which will become part of Toobworld as a direct-to-TV/DVD movie to be shown on the Cartoon Network (even though it will be live action). Not that it matters, as this can stand alone as the representative for the TV Universe.
It's going to be a prequel called "Scooby Doo: In The Beginning" and will be about the Scooby Gang solving mysteries together for the first time (finding out who's been haunting their high school).