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.
The prequel idea is problematic in another way because the Dinkley's museum establishes that all of the classic monsters we're familiar with are already "history" as far as MI's timeline was concerned. "Alice May" took this idea and made an episode out of it (a great one in my book), and for me it implied that the series was going to be a "soft sequel," i.e. only keeping elements that were integral to the series formula, but ditching stuff no one remembers like Coolsville or the ages of the gang (which frankly makes more sense for the 60's gang to have been teenagers rather than adults.)