If Smash followed the ideology of "Recognizability is key," we'd probably have at least three Sonic characters now, among others. The thing is, Smash doesn't really have a consistent line of thinking. We have Steve, the player character of the best-selling game of all time (So far), and Sephiroth, the JRPG villain from the JRPG (For the West, at least), right after Byleth and Min Min, from decently successful Switch released but nowhere near the level of Steve and Sephiroth.
Aside from that, first-parties seem to have lower standards than third-parties. I'd presume it's a mix of A: Not having to go through any negotiations, and B: Being able to promote their own IPs, encouraging a stable loop that gives them the full profits without having to share with any third-parties. Well, it's probably a little more complex than that, but Nintendo seems to be less strict with their own characters than another company's.
But in all honesty... I can't really say who's more likely than who anymore. I have cracked, but in a direction. I've just given up on trying to find precise, accurate results at this point. Now, I'm just gonna name a character go "Yeah, I guess they could happen," and leave it at that.