If they wanted to add more characters, they could've done it before releasing the game. There is absolutely no need for them to release it and then add the characters in as DLC. Especially paid DLC. I will settle for what characters I have unlocked, but I will not buy DLC for anything like that. It is almost always planned just for the profit.
I'd honestly rather wait for the next SSB then pay for more characters in the current one. Once they develop the game, that's it. That's all I'm getting.