On the "prequel" idea, this wouldn't really work as a prequel to the original series, since there they weren't on a cross-country road trip, just living in their hometown. Also, in the mainstream Scooby continuity, the gang lives in Coolsville rather than Crystal Cove, and they have different parents and families than they were shown to have in SDMI. (For instance, Fred's parents are Skip and Peggy Jones, and Daphne doesn't seem to have any sisters.) Maybe you could treat the ending of SDMI as a loose lead-in to the modern What's New, Scooby-Doo?
and DVD movie series, but those contain occasional references to the mainstream continuity so it'd be a rough fit.
Maybe the idea is that now the Mystery Incorporated gang is going to have adventures very much like those of the mainstream Mystery Inc., but not the exact same ones we've seen before.