Tbh I don't see where's the problem with a Mario movie: it's true, Mario was never an interesting protagonist, he was just fitting for the player, that doesn't mean he can't get a personality, nor does it mean he has to be on a journey to save Peach. The only real problem imho is that Charles Martinet MUSN'T talk in his Mario voice for all the movie, shure, he can throw a few "Mamma Mia!" in there but he should also talk in his regular voice to allow for the character to be able to talk freely. Talking about plot almost anything could fit: Mario & Cappy's Funky Holiday, an emotional movie where Mario realizes he doesn't have much to do unless Peach is in danger due to Bowser disappearing (I know, it's Wreck it Ralph, but it's also just an example from someone who isn't paid to make a script ), Bowser sending Mario in an alternative universe where he's the bad guy and Bowser the hero and he has to prove he's not harmful...
