Not just unlikely: this is actually a golden rule of Smash Bros characters right up there with "they must be from a videogame or videogame-related". Not a single secondary character has been added before the protagonist of any franchise.

The only pseudo-exceptions to this rule are Shulk, Cloud and Pikachu, as those franchises have more than one protagonist depending on how you look at them, and even then, the first 2 are the protagonists of their specific game and Pikachu is the undisputed mascot that obviously made it in first.

Well, there's Robin and Lucina, who got in Smash before the main character of their game, but even then that was an special case, considering Fire Emblem is a franchise where each game has their own story with their own characters and that Marth, the most important character of the series as a whole, was already playable, so they being included before Chrom didn't felt weird or anything.