In theory, anyone could do it honestly. For a first game, a 20 something character roster is more than enough and I can't think of any big third party developer that couldn't pull it off.
What Smash got right that most probably can't though is a series of factors that have nothing to do with the roster:
- gameplay, so simple yet satisfying and varied
- style, every character looks like it comes from the same game even if design is completely different (compare Ultimate's Ganondorf and Pikachu to Playstation All-Stars' Kratos and Sly Cooper to see what I mean)
- humor, even "serious" characters like Snake are treated equally to others like Pichu, there is not real favoritism so to speak. Again, look at how Kratos or Big Daddy behave in Playstation All-Stars which is almost cringe inducing. Not really easy to explain, it's just a feeling the game lets off.
- content, Smash has a massive roster now but that's not even the entire thing, I mean the game has a collection of modes, collectables, achievements and stuff that makes certain open world games pale in comparison
Of course all of these things have been possible because Smash is now a 20 years old franchise and became bigger and bigger, but honestly Melee already was massive for its time so I guess it's just Sakurai's talent. As many other things in life Nintendo had luck on its side and struck gold in Sakurai, but you cannot force luck so that's why Playstation All-Stars failed.
The roster per se, while not incredible, was good but the problem with that game was everything else. It was just boring, empty, and completely devoid of charme.
