If you don't add in blood and all that, then it won't be a hardcore game (one more thing. Why are online FPS's the only games that are hardcore? Anything else is casual.)
If you add in the blood and all that, then it wouldn't be a Mario-type game, simply because Mario and his gang are like that.
If there's gonna be an online FPS, don't expect it to include a Nintendo character (Except the red-headed stepchild on the DS)