Here's my idea for a hack that could go in any mario game-
A blue shell, it'd be random, but sometimes it just comes up from behind you, or even straight down out of the sky, slams into you and explodes, killing you, right before the end of the stage. It could happen at any time during a level but it should usually happen right before the end just to make the games really miserable.
The quote "Sometimes life isn't fair. Sometimes in life you have something where you feel that's not right, and that's frustrating." by Nintendo's Kosuke Yabuki should appear in the game(s). In super mario world, all of the info boxes could have that in them. In super mario 3 princess' letters could say it, in smb1 the toads at the end of the castles say it, smb2, maybe it could say it between the levels or something.
The blue shells are just a horrible concept, absolutely terrible, there's no excuse, no justification, and saying life isn't fair is the absolute worst argument that should ever, ever be made in defense of any video game mechanic. We play games to get away from the troubles of real life. I'd like to see hacks that illustrate just how stupid this concept is, when applied to other games. You can play the levels as perfectly as possible, but there's a hidden pair of dice constantly rolling, deciding whether you are allowed to beat the stage or not.
Idiotic. It should be in every mario game.