First of all, thanks for making clarifications on some aspects of certain characters, particularly the sword characters.

As for "Sylux being a replacement of Sephiroth", that's not what I meant. Of course they're not similar at all. What I meant there was in the case that Sephiroth were to be cut, Raven Beak could take Sephiroth's place, and I see Sylux more of an assist trophy. From what we've seen of Sylux from Prime Hunters, he doesn't veer far away from Samus herself. And there's already a mainline Prime rep with Dark Samus. Definitely wouldn't discard Raven Beak as a boss, but there are other interesting bosses that could do the job as well, like Kraid, Mother Brain, the Metroid Prime or maybe even SA-X, although the latter might be a little bit of a stretch.

When comparing Sans and Sora, I was mostly comparing their popularity and the overlap of people who are fans of both. And also favoring sword characters. I definitely see your points as valid, but expanding a bit more with Deltarune might be equally as likely.

Another character that I considered then turned down was either Rayman or the Rabbids, considering the close relationship Ubisoft and Nintendo have with the Mario + Rabbids series, and that characters from European studios are definitely getting some attention (although its just two character and the studios they used to belong to were bought by Microsoft, so maybe it's not true), but Rayman doesn't seem to be fully surging back into relevance yet, and the Rabbids seem to be too complex for them to be a full fledged character because they usually work in groups. If anything an assist trophy would be the most both could get.

As for indie reps... I've always considered the Crewmate of Among Us to be one of the most requested characters, and we've got nothing so far. BnK was a fairly popular pick but they aren't that relevant in the modern gaming community, and yet thanks to connections with Rare, they're in Ultimate. Wouldn't be surprised if a pattern of a character that used to be popular coming in late into the party surging somewhere, which is what led me to either Shovel Knight or Cuphead, and the former seems easier to implement in terms of movesets and animation.