This is backwards thinking. Mario is inherently linear, originally linear, always linear. To keep it linear is not dumbing it down, it's staying true to what the main series has always been: A series of platforming levels. The most important thing is keeping it focused on platforming, because no other game series really even tries to approach that, and of course to keep inventing and staying playful with level design. "Open" does not mean more "ambitious" or forward-thinking when it comes to Mario. Maybe it'd work, but it has nothing to do with Mario' strengths as we know them today. There are plenty of great open world games, but there are no 3D platforming games coming remotely close to delivering on any of the qualities of 3D Mario.