SMB 2 (US) Character Hack Idea:
The US version of Super Mario Bros 2 (for NES) moved itself away from a lot of the other Mario games. Levels aren't linear, no time system, and the game felt more like an adventure that mixed a feeling of SMB and Metroid like exploration.
You even had the ability to select a character to play as for each level, and each one had their own strengths and weaknesses. But therin lies the problem, since quite often you had no idea what the next level would be like, and never had an idea of which character would work best until after you made the selection.
Had you been able to swap characters in and out during the level stages, rather than just at the beginning of each stage, I think it would have been a much more balanced and flavorful experience. Not only would it have solved the issues of coming up to an obstacle that is very difficult to pass with your current character choice, thus creating a situation where you either had to rely on luck or a heavy dose of frustration, it would have felt more like and adventure where all your characters are with you at all times , and allow for some strategy in overcoming specific difficulties.