Now I know you might be thinking "Wait Nintendo's already made 3 Open World Mario Games" but I'm asking your opinion on a hypothetical "Truly" Open World Mario game. By "Truly" I'm talking Breath of the Wild, Skyrim, etc. No hub, no traveling between lands, purely open world Mainline Mario game.
If anyone has that argument in mind, they're wrong. Nintendo hasn't made any open world Mario games. Sure, some of them have more open-ended means to reach a goal, but there's still an overall linear structure in it.
Truly open world games tend to have these components
*RPG-like system of level up and skill trees
*Crafting, get materials from randomly generated items near the player to gather or from enemy drops
*Bandit camps
*Radio towers that expose parts of a map
*Bunch of side missions to do
Mario doesn't have any of these. I've played Legend of Zelda: Breath of the Wild and Horizon Zero Dawn, by the way. Mario's worlds, even the Sand Kingdom in Super Mario Odyssey, don't even begin to match the scale of those games.
I'm for an open-world Mario game but open world was a faddish concept at the time, when it seemed like every game and its mother tried to do open world and many just fail. This is similar to the first-person shooter fad of the day and the battle royal fad that was around from a few years ago up until now. Ubisoft's especially notorious for this, and you'd see these tropes crop up so many times players just got weary about bandit encampments and radio towers in open world games. How would Mario do open world while retaining the Mario identity but also not be so reliant on open world tropes that it's a bog standard Ubisoft open-world game but with a Mario paint?