A few questions to ask, next will be the first chapter.
Q/A:
-Which version of the characters should play Scooby, Velma, Daphne, and Fred. (movie, animated series, etc...)
-Should it stay in the Scooby-Doo universe, or crossover into others?
-Do you want Shaggy to gain powers beyond what he'll start with, like the ones seen in movies. (This would effect where I start the series.)
-Recommendation for start of the series, What's New, Scooby-Doo? (2002–2006), Classic Era ("Where Are You!" 1969–1970), Mystery Incorporated (2010–2013), or Before original timelines.