I'm going to be real, I still think it's a prominence sort of thing. I mean if it were a real deal thing, then why are we getting Characters like K.Rool, Ridley, and Simon and not Waluigi, Ashley, or Lloyd? They're both popular, but they have much more prominence in their franchise as MC's or villians. Echos are out of the question when it does come to prominence though, since Daisy did get in. But I think the newcomers we see may be a bit more involved in their franchises.
Not to dunk on these fans, but I'm just saying that you may want to hold ya'lls breaths for this game since I think that more popular series and more prominent characters are more likely to appear than others. It's kind of the reason that I have a suspension that Deathborn could make it, even if he's not popular at all... Goroh has been hard disconfirmed via AT and Falcon's rivals are few, mostly Black Shadow is popular but only for the reason that he'd be a perfect Echo for Falcon. I do get the feeling that we might get some villains who are complementary to their own franchises that are less popular as well, like Eggman. But then again, the third party rule may only apply to Echoes... which I do hope is false... because why wouldn't they show off Shadow or Black Shadow if they're the most expected out of their franchises tbh. In fact, Dark Samus was also held off from the E3 Direct until Ridley got revealed... so I assume there might be a little trickery going on around some parts...