I'm seeing K. Rool brought up again, so I'm going to mention a trademark ability of his that would be super fun and is canon to his game appearances, but would be absolutely busted in competitive play.
In DKC2 and DK64, K Rool has a stealth ability that makes him invisible while attacking. If this ability was featured in a playable smash bros appearance, he would easily be one of the most busted characters in the game.
Why? Because in competitive play, visual telegraphing is extremely important to counterplaying the opponent. Reactions to the opponent are what make or break certain players from being better than others. Take away a strong character's visuals, and the opponent will have excessive difficulty with counterplay. Even if K. Rool were mediocre or bad and he had stealth, he would still be a decent character. I get the feeling this ability wouldn't be included in his arsenal if he were playable though, but this is all food for thought.