I already know people will get annoyed at this suggestion, but what if a Champion from BotW was planned as DLC?
If you remember, "Zelda Wii U" was being brought up by Nintendo since 2013. There was a lot of hype surrounding that game back then, so what if Sakurai planned for one of the characters as DLC, but didn't know what his options were? Yes, the game wouldn't launch until early 2017, but that's only three and a half months after Su/Mo, the game that Incineroar debuted in.
I already know it's unlikely, seeing how each character comes with a stage and we already have something from BotW. I guess it's not a total deal-breaker since there's other games in Smash that have gotten multiple stages (SM64, KI:U, X/Y), but I don't see it a very realistic outcome. Wouldn't mind being wrong though.