Sonic can do mature subjects far more realistically than Mario. They'd have to totally revamp and ditch most of Mario's core. As for Zelda, I like the lighthearted themes of Wind Waker & Hourglass...however, for the rest of Zelda, they need to take a page from God of War: Bloody fights ÷ bonin' down Zelda = Billions of dollars. Literally. Or maybe just "billion", singular...
