If you've ever played Jump Ultimate Stars or Dream Mix Tv Fighters (or whatever it's called) you would know that simply having popular faces isn't enough to make a game good and make a game sell. On the surface SSB simply seems like a mascot game, but it's much more than that. The game has much more depth than you're giving it credit, and was honestly made successful thanks to its addicting game play.
It may seem like a selfish decision to you, but the fact that the fanbase ignores the simple facts mentioned above prove that few of them know anything about game design or about what makes this game tick.
What I'm saying is that rather than adding Waluigi, Mewtwo or Roy because they're popular, or adding PacMan or Chrom or the Gen 6 Pokemon because they're relevant, I'm going to look at all the available choices and see what each one had thats unique or interesting in regard to gameplay. Sure, popular choices will still get in, but so will obscure ones, thus leading to many pleasant surprises. You are an artist, and the game is your canvas. Call it selfish if you like, but it is you responsibility as a designer to find inspiration in the characters you choose and to make them in such a way that the players can find the same passion you did for the characters you picked. This is the mentality any successful game designer needs to have.
This means no clones, no shortcuts, no halfasses work.
Every character is very unique and interesting in their own right because they are different down to the smallest details.