Quality over quantity.
While speaking to Nintendo Power, Game Designer Masahiro Sakurai argued that more characters isn't what the series needs, and went as far as to say more characters damages balance. He explained:
There is a certain charm to games that have huge casts of playable characters, but they tend to have issues with game balance and it becomes very difficult to fine-tune each character and have them all feel distinctive.
In terms of how that affects the next title, he said:
In terms of quantity, we’ve probably already reached the limit of what’s feasible. I think a change of direction may be what’s needed.
Each of the three previously released games have introduced 10 or more new characters per iteration. The last release, Super Smash Bros. Brawl, featured a sizeable 35 characters, several of which were borrowed from third-party games (i.e. Solid Snake from Metal Gear Solid).
As fun as it was to find out who was being added to Nintendo's frantic fighting series, ensuring that the next release isn't just a copy paste with more characters is essential. The last thing Nintendo needs is another game that feels derivative.