These are game series which have never had a M-Rated title, mainly because they don’t need to! But it would be fun to see just 1 title with an M rating, this is how I think they could do it!
Super Mario Bros.
This is the one that would be the most messed up I think. The graphic violence with the stomping on Goombas and Koopa Troopa’s to death or setting them on fire with the flower. The confrontation between Bowser and Peach would be intense and vulgar laced and the underworld levels would be dark and creepy! The Boos would be terrifying ghosts and the underwater levels would horribly dark with drownings. Plus, you can make Bowser absolutely terrifying!
Sonic The Hedgehog
Just make Dr Robotnik (Not Eggman!) a psychotic mad scientist showing the process of him turning the animals into cyborgs. Have an intense love/hate relationship between Sonic and Knuckles with Tails being conflicted. Plus, showing Sonic killing the villains and not being able to save some of them back into their original organic version.
Instead of having him just eat people make it kind of like Shang Tsung from Mortal Kombat devouring their souls and screaming. Keep Kirby silent with a twisted look on his face and make it graphic whenever he uses any of the abilities to kill enemies like the sword or turning into a weight to crush enemies.
Spyro The Dragon
Really all that needs to be done is have the person being set on fire scream and burn to death graphically and add profanity in the dialogue. Oh, you can also make the levels dark and terrifying like Silent Hill!
Legend Of Zelda
This is by far the easiest to do, just add gore to any of the violence and the same with Kirby make the environments dark and creepy like Silent Hill.
Really just add profanity to the dialogue and any levels on the ground and not in the ships have more graphic kills.
Really just add some graphic content to the hits and add death animations when a character is defeated a-la Mortal Kombat fatalities style.
Make it similar to the Halo and Gears of War series where the kills are graphic and brutal. Also you can add nudity with Samus. Really you have to potential to make a game which as similar to the Aliens movie franchise in terror.