Mario is the kinda guy that, no matter how much you try, you can't see him in a different light. It would seem nearly impossible to make a "Mature" Mario game without losing the charm of the original series. If it ever did come down to this, you can be sure it would probably be a third party developing the game.
That or we'd end up with "Loonatics: Mario version".
On a side note, I wouldn't mind them dealing with stuff like death and complicated situations thats as far as they should go I think.
Kelso: Guys this is a game of Cat & Mouse between me & Hyde. I''m gonna make him admit he''s been going out with Jackie behind my back.
Fez: Guys, what happens in Cat & Mouse if the cat is retarded?