If Smash Bros. just gave us new characters who "fit in the game" every time, that would be lame.
The Ice Climbers are two characters controlled at once, one following the other.
Mr. Game and Watch is what a cartoon person looks like on a tiny 1980s LCD screen, complete with frame-by-frame movement.
Pokemon Trainer stands behind the stage and switches out three Pokemon with their own stamina.
Snake chokes people, plants mines, and shoots missiles for his smashes.
Every one of these characters, people initially went "No way, that wouldn't work." And then when they were revealed people went "Wow. This is awesome. I can't believe they made this work."
The whole series is all about choosing characters who have no business being in a game together, and making it work.
And for the 4th Smash Bros., they are going to have to up the ante again. 3rd party characters from Capcom or Square won't be shocking, and they'll be onto the 4th string of Nintendo characters. They are going to need the likes of Master Chief to get people psyched for the game. In 6 or 7 years, when the next Smash is on it's way, I wouldn't be surprised if Nintendo went out of their way to use characters who HAVEN'T been on a Nintendo platform, ever.
"[Our former customers] are unable to find software which they WANT to play."
"The way to solve this problem lies in how to communicate what kind of games [they CAN play]."
Satoru Iwata, Nintendo President. Only slightly paraphrased.